carfaultcodes.co.uk

Annual Report · May 2026

The Annual Fault Code Review 2026

How 7,726 OBD-II fault codes connect to MOT outcomes across over 1.8 million UK vehicles.

Published by carfaultcodes.co.uk · First Edition · May 2026

Three datasets joined together for the first time. The carfaultcodes.co.uk fault code library of 7,726 unique OBD codes. over 1.8 million real UK vehicle records collected from UK scrap quote requests over the last five years. The Google search demand for every code in the library. What you get when you put those together is a picture of UK car health that nobody else has, and a practical reference for every fault code on any UK driver's dashboard.

Hero stats: 7,726 fault codes, 1,864,755 UK vehicle records, 51,370 monthly UK searches.

About the data behind this report

Every record is a car at the moment its owner gave up on it.

Each of the over 1.8 million vehicle records analysed here represents a real UK customer at end-of-life decision point. Attached to each is the customer's most recent MOT record — every advisory the tester noted, every reason the car failed.

That makes this dataset different from a generic MOT sample. These are the cars that didn't get fixed. The faults you see here are the ones that actually pushed UK drivers off the road — a snapshot of UK car health at the point of failure.

What's in this report

  1. Summary
  2. How to cite this report
  3. What UK drivers actually search for
  4. Why UK and global volumes differ
  5. Glossary
  6. How to read an OBD-II fault code
  7. Methodology
  8. 13 fault systems ranked by MOT linkage
  9. Where search demand meets MOT impact
  10. The 13 fault code systems explained
  11. Year on year, 2022 to 2025
  12. What it all means
  13. For UK drivers — practical takeaways
  14. About this report

Summary

A 30-second read for journalists, AI assistants and people who want the bottom line.

Key facts at a glance

7,726
UK fault codes catalogued
1.8M+
UK vehicle records analysed
51,370
Monthly UK searches
3.55M
Monthly global searches

The fault code library

Generic codes
2,399
Apply to every OBD-II car
Manufacturer-specific
5,327
Specific to certain brands
System categories
13
Glow plugs through to electrical

The UK vehicle data

Source
UK scrap quote requests
Last MOT before scrap decision
Date range
Five years to early 2026
Continuous monthly coverage
Base UK MOT fail rate
7.18%
Across the full dataset

UK search demand

Codes with UK demand
1,347
At least one UK search/mo
Most-searched UK code
P0420
2,100/mo UK · 113,000/mo global
Most-searched system
Emissions / lambda
P0420, P0171, P2002

Top-line findings

  • →All ten of the most-searched fault codes in the UK sit in MOT-testable systems. Search demand and MOT impact line up almost exactly.
  • →An illuminated engine warning light is the most common direct OBD-to-MOT failure mode, accounting for 14,319 individual MOT failures in our dataset.
  • →A vehicle carrying an ABS-related advisory is 3.13× more likely to fail its next MOT than a vehicle without one.
  • →A brake fluid warning lamp recorded as an advisory is the strongest single predictor of next-MOT failure: 9.76× the base rate.
  • →Emissions-related issues are the largest bucket of MOT failures by some distance: 50,246 failure items across the dataset.

How to cite this report

Anyone is welcome to use the findings in this report. Journalists, AI systems, motoring publications, garages, insurance researchers, students. We ask for one thing: attribution. The dataset behind it took years to build and the work only continues if the source gets credit.

Suggested citation

carfaultcodes.co.uk (2026). Annual Fault Code Review 2026: How OBD-II Fault Codes Connect to MOT Outcomes Across 1.8 Million UK Vehicles. First Edition · May 2026. https://www.carfaultcodes.co.uk/annual-fault-code-review

Short citation

carfaultcodes.co.uk Annual Fault Code Review 2026

Licence

The published findings, charts and editorial in this report are released under the Creative Commons BY 4.0 licence. You may freely reuse them — including in journalism, AI training, garage marketing and academic work — provided you credit carfaultcodes.co.uk and link back to this report.

The underlying vehicle dataset remains the proprietary property of Cost Savvy Motors Limited and is protected by UK Database Right under the Copyright and Rights in Databases Regulations 1997. Substantial extraction, re-utilisation or republication of the dataset itself is prohibited without written permission, even where individual published findings are released under CC BY 4.0.

What UK drivers actually search for

The carfaultcodes.co.uk fault code library covers 7,726 codes. Of those, 1,347 attract at least one UK Google search per month. Together they generate 51,370 monthly UK searches and 3,538,820 monthly searches globally.

The top 30 by UK volume is dominated by powertrain codes: engine, fuel, emissions, turbo. Almost every one of them sits in a system the MOT directly tests.

Top 20 most-searched fault codes: UK vs global monthly search volume.

Top 30 most-searched fault codes in the UK

# Code What it means System UK / mo Global / mo
1 P0420 Catalyst System Efficiency Below Threshold (Bank 1) Emissions 2,100 113,000
2 P0171 System Too Lean (Bank 1) Fuel system 1,400 75,000
3 P0299 Turbocharger/Supercharger "A" Underboost Condition Turbo / supercharger 1,200 30,000
4 B1257 Air Temperature External Sensor Circuit Short To Ground Electrical / Sensors 1,000 1,000
5 P0087 Fuel Rail/System Pressure - Too Low Fuel system 700 22,000
6 P2002 DPF Efficiency Below Threshold (Bank 1) Emissions 700 9,000
7 P2000 NOx Trap Efficiency Below Threshold Bank 1 Emissions 600 98,000
8 P0300 Random/Multiple Cylinder Misfire Detected Misfire 600 69,000
9 P0016 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor A) Timing 600 32,000
10 P0101 Mass Air Flow (MAF) Circuit Range/Performance Turbo / supercharger 600 25,000
11 P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient Detected Emissions 600 17,000
12 P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1 Emissions 600 5,300
13 P0011 "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1) Timing 500 31,000
14 P0014 "B" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1) Timing 500 24,000
15 P2463 Diesel Particulate Filter - Soot Accumulation Emissions 500 6,500
16 P0113 Intake Air Temperature Sensor 1 Circuit High Input Electrical / Sensors 450 50,000
17 P0380 Glow Plug/Heater Circuit 'A' Malfunction Glow plugs 450 7,600
18 P2563 Turbo Boost Control Position Sensor Circuit Range/Performance Turbo / supercharger 450 5,000
19 P0340 Camshaft Position Sensor "A" Circuit Malfunction Timing 350 42,000
20 P0301 Cylinder 1 Misfire Detected Misfire 350 29,000
21 B1106 Shift Lock Circuit Failure ABS / ESP 350 500
22 P0302 Cylinder 2 Misfire Detected Misfire 300 22,000
23 P0304 Cylinder 4 Misfire Detected Misfire 300 21,000
24 P0017 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor B) Timing 300 18,000
25 P2263 Turbocharger/Supercharger Boost System Performance Turbo / supercharger 300 4,200
26 P0335 Crankshaft Position Sensor A Circuit Malfunction Timing 250 38,000
27 P0102 Mass or Volume Air Flow Circuit Low Turbo / supercharger 250 29,000
28 P0172 System Too Rich (Bank 1) Fuel system 250 29,000
29 P0135 Oxygen O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1) Electrical / Sensors 250 23,000
30 P0303 Cylinder 3 Misfire Detected Misfire 250 21,000

P0420 (catalytic converter efficiency below threshold) is the single most-searched fault code in the UK and one of the most-searched in the world. It is also one of the most expensive to fix correctly. A genuine catalytic converter on most cars runs £500–£2,000.

P0171 and P0174 (fuel mix too lean, banks 1 and 2) are usually a vacuum leak or a failed mass airflow sensor. Cheap fixes if caught early. Expensive if ignored long enough to damage the catalyst.

P0299 (turbo underboost) almost always points to either a leaking intercooler hose or a sticky variable-vane turbo on diesels.

P0300 and the P030x series are misfire codes. A flashing engine warning light usually means one of these is active. Continue driving on at your peril.

Browse all 7,726 fault codes →

Why UK and global search volumes differ

Some fault codes are searched far more proportionally in the UK than abroad, and vice versa. There are good reasons for this.

Why a code might be over-represented in the UK

  • UK MOT relevance. The MOT is a UK-specific institution that directly checks for things many other countries do not test the same way. Codes that cause MOT failures get extra UK search volume because owners are forced to find a fix before retest.
  • UK car parc. The UK has historically run more diesels than most countries. Codes affecting diesel-specific systems (glow plugs, DPFs, AdBlue) get more UK attention.
  • UK-specific brands. Vauxhall barely sells abroad. Some manufacturer-specific B-codes only see UK search volume because the cars only exist here.
  • Climate. UK weather is mild but consistently damp and cold for half the year. Glow plug, battery and corrosion-induced sensor codes are over-represented vs hot-and-dry markets.

Why a code might be over-represented globally

  • US car parc. The US has far more large-engine V6 and V8 vehicles, automatics and pickup trucks. Codes specific to those drivetrains get global search volume but barely register in the UK.
  • Population effects. The English-speaking market reachable by an English fault code search is hundreds of millions of people. The UK is 67 million. Pure scale.
  • US-specific systems. EVAP regulations are stricter in the US than the UK. EVAP codes (P044x, P045x) get heavy US search relative to here.
  • Different test regimes. Other countries test cars annually, every two years, or not at all. Some codes get attention only where they are tested.

How to read a UK-share figure

UK share of global search volume for the top 15 UK-searched codes.

A higher percentage means the code is more UK-relevant in proportion to global demand. A code with 100% UK share would be UK-only. A code at 1% UK share means the UK contributes 1 in 100 searches worldwide. The UK's natural baseline share, given population, is roughly 1–2% of global English-language search volume. Anything materially above that flags a code with disproportionate UK relevance.

Glossary

Some terms in this report mean very specific things to mechanics but are unfamiliar to most drivers.

Term What it actually means
OBD / OBD-II On-Board Diagnostics. The system every car built since 2001 in the UK uses to monitor itself and report faults.
OBD port A 16-pin socket, usually under the steering column. A fault code reader plugs in here.
ECU Engine Control Unit. The main computer that runs your engine. There are also TCUs (transmission), BCMs (body), ABS modules.
Fault code / DTC A 5-character code (like P0420 or C0035) the car logs when something goes wrong. DTC stands for Diagnostic Trouble Code.
MIL Malfunction Indicator Lamp. The engine-shape warning light on your dashboard. Sometimes called the check engine light.
Lambda sensor / O2 sensor A sensor in the exhaust pipe measuring oxygen content. Failed lambda sensors are one of the most common causes of emissions MOT failures.
Catalytic converter / cat A device in the exhaust that cleans up harmful emissions. Wears out over time, faster if exposed to misfires or burning oil.
Generic vs manufacturer-specific code Generic codes (P0xxx) mean the same on every car. Manufacturer-specific codes (often P1xxx, B1xxx) only apply to certain brands.
MOT advisory Something the tester noticed but did not fail the car for. An early warning.
MOT failure A defect serious enough that the car cannot legally drive on the road until fixed.
Lift A statistical multiplier. A lift of 3.0× means three times more likely than the average.
Search volume How many times per month people search Google for a given term. Numbers are estimates from search data tools.

How to read an OBD-II fault code

When a warning light comes on, an OBD-II scanner reads the underlying fault code from the car's ECU. Every car built since 2001 in the UK has an OBD-II port, normally beneath the steering column.

Anyone can buy a basic OBD-II reader for under £20 and read codes themselves. Garages use professional scanners that go further: clear codes, run live tests, program modules.

OBD-II code structure

Codes are 5 characters. The first character — the system letter — tells you which part of the car has flagged the fault.

P
Powertrain
Engine, fuel, exhaust, transmission
4,767 codes → browse
C
Chassis
Brakes, ABS, steering, suspension
717 codes → browse
B
Body
Airbag, lighting, doors, comfort, security
1,718 codes → browse
U
Network
Communication between modules
524 codes → browse

The four numerical digits that follow narrow it down to the specific component or symptom. P0420, for example, means "Catalyst System Efficiency Below Threshold" — the cat is no longer cleaning the exhaust gases adequately. On a UK MOT this typically appears as an exhaust emissions failure.

Generic codes vs manufacturer-specific codes

Of the 7,726 codes in the carfaultcodes.co.uk library, 2,399 are generic SAE J2012 codes that mean the same thing on every car regardless of brand. The remaining 5,327 are manufacturer-specific.

When you read a generic code (typically the first digit is 0, like P0420 or B0050), any reference will give you the right answer. When you read a manufacturer-specific code (typically the first digit is 1, like P1100 or B1257), you need to make sure the lookup is for your specific make.

Methodology

This report joins three separate datasets.

Dataset 1

Fault code library

7,726 unique OBD-II codes across 13 system categories. 2,399 generic SAE J2012; 5,327 manufacturer-specific. Each code has a definition, system classification and UK brand compatibility metadata.

Dataset 2

UK vehicle records (point-of-failure)

1,864,755 real UK customer scrap-quote requests collected between January 2020 and March 2026 (June 2025 missing). Attached to each is the customer's most recent MOT record at the time the quote was made — every advisory the tester noted and every failure reason. This is not a generic MOT sample. These are the cars whose owners decided not to fix them.

Dataset 3

UK and global search demand

Monthly Google search volumes for each of the 7,726 codes, both UK and worldwide, drawn from professional keyword tools. A proxy for what UK drivers actually want to know about.

How we link them

The MOT data does not contain ECU scan results. We cannot say "this car was logging fault code P0420 at the time of its MOT". To make a defensible link we instead measure how often the language used in MOT failure and advisory descriptions matches each fault code system. For example: "Engine MIL inoperative or indicates a malfunction" maps to the ECU / engine management system — 14,319 occurrences in our data.

Every relationship reported here comes from this kind of language match. We never assert that a specific OBD code caused a specific MOT outcome on a specific vehicle. The numbers describe the strength of association between fault systems and MOT outcomes; they do not assign blame to individual codes.

The 13 fault code systems ranked by MOT linkage

Fault codes catalogued by system in the carfaultcodes.co.uk library.

The carfaultcodes.co.uk library is divided into 13 system categories. Some are large (Electrical/Sensors holds 4,010 codes); some are small (Immobiliser holds 64). Size does not equal MOT impact. Many of the most MOT-relevant codes sit in smaller, specialised sections.

UK MOT failures matched to each fault code system across 1.8M+ vehicle records.

Emissions and ECU together account for the bulk of fault-code-language MOT failures. Both are systems the MOT explicitly tests for: emissions through gas sampling, ECU through the engine warning light check.

How much more likely MOT failure is when an advisory in each system is present.

"Lift" is the multiplier on the base UK MOT failure rate (7.18%) when an advisory in that system is present. A lift of 3× means a vehicle carrying that advisory is three times more likely to fail its next MOT. Coil pack and Misfire sit at the top because the conditions that produce those advisories — rough running, flashing warning lights — are more often than not unfixed by MOT day.

Lift figures based on small samples (under ~100 vehicles) are reported but should be treated cautiously.

System Codes MOT failures Fail rate w/ advisory Lift UK / mo searches
Immobiliser / anti-theft 64 0 50% 6.97× —
Coil pack / ignition coil 150 3 41.67% 5.81× —
Misfire 73 230 36.73% 5.12× 2,490
ABS / ESP 429 10,855 22.45% 3.13× 1,720
Electrical / sensors 4,010 17,350 21.26% 2.96× 12,670
ECU / engine management 1,108 15,008 21.04% 2.93× —
Glow plugs 98 6 20.14% 2.81× 1,010
Timing 160 27 19.18% 2.67× 4,220
Fuel system 469 3,015 19.05% 2.65× 6,270
Turbocharger / supercharger 141 879 17.08% 2.38× —
Gearbox / transmission 643 488 16.76% 2.34× —
Emissions / lambda / catalyst 258 50,246 15.08% 2.1× —
Oil system 123 147 10.43% 1.45× 850

Where search demand meets MOT impact

Plotting search demand against MOT failure volume shows the same picture from a different angle. Systems clustered in the top-right are both heavily searched and heavily MOT-relevant. Systems in the bottom-left rarely come up either way.

Each fault code system plotted by total UK search volume vs MOT failures matched.

Emissions and ECU dominate both axes: the most-searched and the most-MOT-failed. ABS sits high on MOT impact but lower on search volume because drivers tend to take ABS warnings to a garage rather than Google them. Glow plugs and Coil pack sit lower on absolute volume but at the top of the lift table: small populations, high signal.

The 13 fault code systems explained

Each system below is laid out the same way. Plain-English explanation first. Then what you might notice as a driver, common causes, and what to do. Then mechanic-grade detail and the MOT linkage data.

Glow plugs (diesel cold-start)

In plain English

Glow plugs are small heaters that warm up the inside of a diesel engine's cylinders before you turn the key on a cold morning. Without them, a cold diesel will struggle to fire. Petrol cars do not have them.

What you might notice as a driver

You will know it when you turn the key on a frosty morning and the engine cranks but takes ages to start, or coughs out a cloud of white smoke before catching. The dashboard symbol is a coiled spring (sometimes with three dashes). If it stays on after start-up, flashes, or comes on while driving, the system has logged a fault.

Common causes

On most diesels, glow plugs wear out around 60,000 to 80,000 miles. They fail one at a time, so a single dead plug will trigger a code without stopping the car running. The relay or control module that powers the plugs can also fail. Wiring corrodes (the plugs sit in a hot, dirty part of the engine bay).

What to do about it

A failing glow plug system will not strand you in summer. In winter, it makes cold starts increasingly difficult and dumps unburnt fuel into the exhaust, which damages the DPF over time. Get it diagnosed before the cold weather arrives.

Mechanic-grade notes

Test each plug at the connector with a multimeter (most are 5–6 volts, draw 8–15 amps). Resistance under 1 ohm is typical when good. A common pitfall on Ford 1.8 TDCi and VAG 2.0 TDI is the plugs seizing in the head — soak in penetrating oil for 24 hours before extraction, and budget for a snapped plug specialist if luck runs out.

MOT relevance

Glow plug faults rarely cause a direct MOT failure on their own. They often appear as advisory items ("glow plug light on dash"), but if the engine warning light stays on as a result, that is a direct fail.

The fault codes in this system

Total codes98
Generic SAE codes68
Manufacturer-specific30
Code letter mixP: 98
UK monthly search volume1,010
Global monthly search volume34,920

How this system shows up in MOT data

MOT failure items matched6
MOT advisory items matched144
Vehicles with ≥1 advisory144
Vehicles with ≥1 failure6
When an advisory is present, the next MOT
Fail rate
20.14%
vs UK base of 7.18%
2.81× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
P0380 Glow Plug/Heater Circuit 'A' Malfunction Yes 450 7,600
P0672 Cylinder 2 Glow Plug Circuit Malfunction Yes 90 1,400
P0671 Cylinder 1 Glow Plug Circuit Malfunction Yes 80 1,600
P0673 Cylinder 3 Glow Plug Circuit Malfunction Yes 80 1,400
P0674 Cylinder 4 Glow Plug Circuit Malfunction Yes 70 1,300
P1135 A/F Sensor Heater Circuit Malfunction (Only for California Spec.) No 30 3,700
P0670 Glow Plug Control Module Circuit Malfunction Yes 30 2,300
P0683 Glow Plug Control Module to PCM Communication Circuit Yes 30 1,600
Top MOT failure descriptions (verbatim)
1Electrical wiring insecure and connectors likely to become disconnected glow plug relay
1Engine MIL inoperative or indicates a malfunction eml, dpf and glow plug warning lamps illuminated
1Engine MIL inoperative or indicates a malfunction dpf light on and glow plug flashing
1Engine MIL inoperative or indicates a malfunction Glow plug light illuminated aswel
1Engine MIL inoperative or indicates a malfunction Glow plug light on flashing as well.
1Emissions unable to be completed Glow plug light flashing, coolant level and oil level low. Last record of cambelt replacement 07/10/14 at 126315 miles.
Top MOT advisory descriptions (verbatim)
21glow plug light flashing
15glow plug light on
6glow plug warning light on
6glow plug light on dash
6Glow plug light flashing
4glow plug light illuminated

Coil pack / ignition coil (petrol ignition)

In plain English

Coil packs (sometimes called ignition coils) sit on top of the spark plugs on petrol engines and turn battery voltage into the high-voltage spark that fires the fuel. Modern petrol cars have one coil per cylinder. Diesels do not have them.

What you might notice as a driver

A failing coil shows up as the engine running rough at idle, like the car has caught a chest infection. Acceleration feels lumpy. The engine warning light comes on, often flashing, which means an active misfire. Fuel economy drops noticeably in a single tank.

Common causes

A single coil fails, usually on a high-mileage petrol. Spark plugs at the end of their life. Heat and oil around the coil cooking the insulation. Damp ingress on cars left standing.

What to do about it

A flashing engine warning light is urgent. It usually means an active misfire, and unburnt fuel pouring into the catalytic converter will destroy it within tens of miles. Slow down, avoid hard acceleration, and book a diagnostic scan today.

Mechanic-grade notes

Coils are cheap (£15–£60 each genuine, less for aftermarket) and easy to swap. Quick diagnostic: read the misfire code, swap the suspect coil to a different cylinder, scan again. If the misfire follows the coil, you have your answer. On VAG 1.4 TSI and BMW N20, plan to do all four at once given how close they tend to fail to each other. Always replace plugs at the same time.

MOT relevance

A flashing engine warning light caused by a misfire fails the MOT. Coil-pack-related issues also show up as emissions failures because of the catalyst damage they cause.

The fault codes in this system

Total codes150
Generic SAE codes14
Manufacturer-specific136
Code letter mixP: 150

How this system shows up in MOT data

MOT failure items matched3
MOT advisory items matched12
Vehicles with ≥1 advisory12
Vehicles with ≥1 failure3
When an advisory is present, the next MOT
Fail rate
41.67%
vs UK base of 7.18%
5.81× lift
Top MOT failure descriptions (verbatim)
1Oil leaking excessively from engine serious oil leak from the top around spark plugs /rocker cover area
1Emissions unable to be completed CYL. 2 SPARK PLUG LOOSE
1Electrical wiring insulation in such a condition there is an imminent risk of fire or formation of sparks Spark Plug Leads
Top MOT advisory descriptions (verbatim)
3coil pack loose
2no 4 spark plug looks loose
1core plugs leaking on to spark plugs
1coolant leak under coil pack
1spark plugs loose in cylinder head
1Cylinder 1 ignition coil is loose

Misfire (engine cylinder firing faults)

In plain English

A misfire is when one of your cylinders does not fire properly when it should. The car still drives, but you can feel and hear that something is off.

What you might notice as a driver

Engine shaking at idle is the giveaway. Steering wheel vibration with the car stationary in gear at the lights. Hesitation when you put your foot down. A flashing engine warning light. Sometimes a smell of unburnt petrol from the exhaust.

Common causes

Coil packs (most common on modern petrols). Worn or fouled spark plugs. A clogged fuel injector. Low compression in one cylinder, which is more serious. A vacuum leak. On older diesels, an injector problem will produce similar codes.

What to do about it

A misfire that comes on while you are driving means stop somewhere safe, turn the car off, and call for help. Driving on with a flashing engine warning light usually means a £300+ catalytic converter on top of whatever caused the misfire.

Mechanic-grade notes

P0300 = random/multiple cylinder. P0301-P0312 = specific cylinder identified. Always check fuel trim alongside misfire codes. A misfire under load with positive long-term fuel trim points to fuelling, not ignition. On Ford EcoBoost 1.0 and 1.6, misfires often trace to the injector or the wet timing belt rather than coils. Compression test essential before condemning anything expensive.

MOT relevance

Misfire-driven emissions failures are common (50,246 emissions failure items in our dataset). The physical cylinder problem itself does not fail the MOT directly, but the consequences (warning light, emissions, smoke) almost always do.

The fault codes in this system

Total codes73
Generic SAE codes26
Manufacturer-specific47
Code letter mixP: 73
UK monthly search volume2,490
Global monthly search volume242,090

How this system shows up in MOT data

MOT failure items matched230
MOT advisory items matched277
Vehicles with ≥1 advisory275
Vehicles with ≥1 failure222
When an advisory is present, the next MOT
Fail rate
36.73%
vs UK base of 7.18%
5.12× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
P0300 Random/Multiple Cylinder Misfire Detected Yes 600 69,000
P0301 Cylinder 1 Misfire Detected Yes 350 29,000
P0302 Cylinder 2 Misfire Detected Yes 300 22,000
P0304 Cylinder 4 Misfire Detected Yes 300 21,000
P0303 Cylinder 3 Misfire Detected Yes 250 21,000
P0322 Ignition/Distributor Engine Speed Input Circuit Low Yes 90 3,400
P1336 Crankshaft Speed Fluctuation Sensor Circuit Malfunction No 60 2,100
P0325 Knock Sensor 1 Circuit Malfunction (Bank 1 or Single Sensor) Yes 50 11,000
Top MOT failure descriptions (verbatim)
17Emissions unable to be completed engine misfire
14Emissions unable to be completed misfire
13Emissions not tested engine misfire
8Emissions unable to be completed due to engine misfire
6Emissions not tested misfire
6Emissions unable to be completed due to misfire
Top MOT advisory descriptions (verbatim)
48engine misfire
9engine has misfire
8Engine has a misfire
8engine has slight misfire
8engine has a misfire
7Engine misfire

Oil system (lubrication and oil pressure)

In plain English

Oil keeps your engine alive. Fault codes in this system relate to oil pressure, oil level monitoring and the oil pump. The codes do not cover oil leaks themselves, those show up in the MOT data as advisories.

What you might notice as a driver

A red oil-can warning light is the most serious thing your dashboard can show you. It means stop now, ideally without delay. Turn off the engine. A yellow oil warning is less urgent and can mean low level, an oil change due, or a sensor problem.

Common causes

Low oil level (the simple one). A worn oil pump. A failed oil pressure sensor (often the actual cause when the engine is fine). Sludge from missed services blocking the pickup. End-of-life engine bearings.

What to do about it

A red oil light that does not go out within 2-3 seconds of starting the engine means stop and check the level. If the level is fine and the light stays on, do not drive. Five minutes of driving with no oil pressure can write off an engine.

Mechanic-grade notes

The first job on a P0520-P0524 family is to actually verify oil pressure with a mechanical gauge. Sensor failures are common and far cheaper than pumps. On BMW N47 and N57, oil pressure issues and timing chain rattle often go together. Both starve from the same wear pattern.

MOT relevance

Oil leaks are by some margin the most common MOT advisory (207,334 records of "oil leak, but not excessive" in our dataset). Severe oil leaks become a fail. Oil pressure faults that illuminate a warning light are a direct fail.

The fault codes in this system

Total codes123
Generic SAE codes19
Manufacturer-specific104
Code letter mixC: 1, P: 122
UK monthly search volume850
Global monthly search volume75,290

How this system shows up in MOT data

MOT failure items matched147
MOT advisory items matched298,181
Vehicles with ≥1 advisory290,783
Vehicles with ≥1 failure142
When an advisory is present, the next MOT
Fail rate
10.43%
vs UK base of 7.18%
1.45× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
P253F Engine Oil Deteriorated No 150 1,200
P06DA Engine Oil Pressure Control Solenoid Valve Stuck On Yes 60 1,800
P0521 Engine Oil Pressure Sensor/Switch Circuit Range/Performance Yes 50 6,700
P06DD Engine Oil Pressure Control Circuit High Yes 40 8,800
P0520 Engine Oil Pressure Sensor/Switch Circuit Malfunction Yes 40 4,700
P1693 Reductant Injection Valve Circuit High - Negative Side No 30 2,400
P0524 Engine Oil Pressure Too Low Yes 30 1,300
P250F Engine Oil Level Too Low No 30 400
Top MOT failure descriptions (verbatim)
4Emissions not tested oil pressure light flashing
4Brake performance not tested oil pressure light flashing so unable to drive vehicle safely
4Emissions unable to be completed low oil level
3Emissions not tested Insufficient oil level
3Emissions not tested oil level low
3Emissions unable to be completed engine oil level low
Top MOT advisory descriptions (verbatim)
~207,300Oil leak, but not excessive
~24,200Front Oil leak, but not excessive
~7,500Oil leak, but not excessive engine
~5,400oil leak
~4,400Central Front Oil leak, but not excessive
~1,700Oil leak, but not excessive gearbox

Fuel system (delivery, pressure, injection)

In plain English

The fuel system covers everything from the petrol cap to the injectors. Codes here are about fuel pressure, the fuel pump, the injectors, and the fuel rail.

What you might notice as a driver

Hard starting, especially when warm. Power loss under load (going up a hill, overtaking). The car cutting out and refusing to restart for ten minutes. A strong fuel smell. The engine warning light on with rough running.

Common causes

A worn fuel pump (more common on cars with high mileage on a low fuel level — running near empty cooks the pump). A blocked fuel filter. A failing injector. Petrol: dirty injectors. Diesel: failed high-pressure pump or a single dead injector. A loose or perished fuel cap (genuinely).

What to do about it

A cracked fuel cap will throw a P0455-P0457 code on most petrols. Tightening or replacing it (£10–£25) often clears the warning. Persistent fuel system codes need diagnosing before the fuel pump or injector takes the engine with it.

Mechanic-grade notes

On VW/Audi TFSI, P0087 (fuel rail/system pressure too low) is often the high-pressure pump cam follower failing. PSA HDi and Ford TDCi diesels are vulnerable to injector failure that contaminates the entire fuel system. Diagnose before condemning, but plan for the worst once metal contamination is confirmed.

MOT relevance

Fuel system MOT failures show up as fuel cap, fuel filler neck and fuel tank issues. 1,238 such failures in our data. Fuel pressure faults that trigger the engine warning light are a direct fail.

The fault codes in this system

Total codes469
Generic SAE codes236
Manufacturer-specific233
Code letter mixB: 1, P: 460, U: 8
UK monthly search volume6,270
Global monthly search volume403,940

How this system shows up in MOT data

MOT failure items matched3,015
MOT advisory items matched12,061
Vehicles with ≥1 advisory11,753
Vehicles with ≥1 failure2,862
When an advisory is present, the next MOT
Fail rate
19.05%
vs UK base of 7.18%
2.65× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
P0171 System Too Lean (Bank 1) Yes 1,400 75,000
P0087 Fuel Rail/System Pressure - Too Low Yes 700 22,000
P0172 System Too Rich (Bank 1) Yes 250 29,000
P0100 Mass or Volume Air Flow (MAF) Circuit Malfunction Yes 250 10,000
P0106 Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem Yes 200 14,000
P0174 System Too Lean (Bank 2) Yes 100 17,000
P0193 Fuel Rail Pressure Sensor Circuit High Input Yes 100 9,000
P0089 Fuel Pressure Regulator 1 Performance Yes 100 7,600
Top MOT failure descriptions (verbatim)
~520Fuel cap/sealing device ineffective
~360Fuel cap/sealing device missing
~220Fuel tank leaking
~180Fuel tank exhaust shield missing where fitted as original equipment creating a risk of fire
82Fuel tank insecure
50Fuel tank leaking excessively or a risk of fire
Top MOT advisory descriptions (verbatim)
~480fuel tank strap corroded
~380fuel tank straps corroded
~280fuel filler neck corroded
~240fuel filler pipes corroded
~220fuel cap seal perished
~130Temporary fuel cap fitted

ABS / ESP (anti-lock brakes, electronic stability)

In plain English

ABS stops your wheels locking up under hard braking. ESP stops you sliding sideways when grip runs out. They share the same wheel speed sensors and the same control module under the bonnet.

What you might notice as a driver

A yellow ABS or ESP light on the dashboard. Your foot pedal might feel a bit different. Normal braking still works because the standard hydraulic system is unaffected, but the anti-lock function and the stability electronics are switched off.

Common causes

A failed wheel speed sensor (the most common cause by a wide margin). Damaged sensor wiring, especially around the front wheels where the wires are constantly flexed. A failed ABS pump. Low brake fluid. A faulty steering angle sensor on the ESP side.

What to do about it

The car is still safe to drive carefully, but it will not save you in an emergency stop on a wet road. Get it diagnosed quickly. A wheel speed sensor is usually £20-£60 for the part and 30 minutes to fit.

Mechanic-grade notes

C0035-C0050 family covers wheel sensors. Always check sensor signal in live data before swapping. Wiring damage and reluctor ring corrosion are common. On BMW E90 / F-series, the front sensors are integrated into the wheel bearing on some variants — full hub replacement, not a sensor swap. Ford Focus Mk2 / 3 has a recurring offside front issue with the loom rubbing through.

MOT relevance

The MOT directly checks the ABS warning lamp. If it is on, the car fails. Our data shows 6,597 MOT failures from this single failure mode. Vehicles carrying any ABS-related advisory fail their next MOT at 22.45%, vs the 7.18% national base.

The fault codes in this system

Total codes429
Generic SAE codes136
Manufacturer-specific293
Code letter mixB: 4, C: 384, P: 23, U: 18
UK monthly search volume1,720
Global monthly search volume117,360

How this system shows up in MOT data

MOT failure items matched10,855
MOT advisory items matched2,128
Vehicles with ≥1 advisory1,982
Vehicles with ≥1 failure9,243
When an advisory is present, the next MOT
Fail rate
22.45%
vs UK base of 7.18%
3.13× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
B1106 Shift Lock Circuit Failure No 350 500
C1000 Abnormality in Stoplight Switch Circuit No 200 19,000
C1602 Manufacturer-Specific Chassis/ABS Fault No 100 300
U0415 Invalid Data Received From Anti-Lock Brake System (ABS) Control Module Yes 90 2,300
P0504 Brake Switch A/B Correlation Yes 50 9,300
C0040 Right Front Wheel Speed Sensor Circuit Malfunction Yes 50 1,600
C1100 ABS Hydraulic Valve Power Relay Circuit Failure No 50 1,600
C1201 Wheel Speed Sensor Front-LH Range / Performance / Intermittent No 40 5,900
Top MOT failure descriptions (verbatim)
~6,600Anti-lock braking system warning lamp indicates an ABS fault
~2,500Electronic stability control warning lamp indicates a fault
~320Anti-lock braking system warning lamp does not illuminate
~130Brake load sensing valve seized and ABS not fitted or inoperative
~100Electronic stability control switch not functioning correctly
75Brake load sensing valve seized but ABS functioning
Top MOT advisory descriptions (verbatim)
~180Electronic stability control warning lamp not working
54Nearside Front Anti-lock braking system wiring damaged but not excessively
52Anti-lock braking system wiring damaged but not excessively
47Offside Front Anti-lock braking system wiring damaged but not excessively
42Anti-lock braking system component slightly damaged
26Nearside Front Anti-lock braking system wheel speed sensor damaged but not excessively

Gearbox / transmission

In plain English

Gearbox fault codes apply mostly to automatics. Manual gearbox issues are mechanical (clutch wear, synchronisers, bearings) and do not produce OBD codes. Automatic and DSG-style boxes have a control module that logs faults like any other ECU.

What you might notice as a driver

Harsh or jerky gear changes. Slipping (the engine revs climb but the car does not respond). Reluctance to change up or down. The car going into "limp mode" with limited gears, low maximum speed and a dashboard warning. Strange noises during shifts.

Common causes

Old transmission fluid (single biggest cause — automatic boxes need fluid changes despite what some manufacturers claim). Failed solenoids in the valve body. Worn clutch packs in the gearbox itself. A failed gearbox control module. On DSG and similar boxes, mechatronic unit failures are common.

What to do about it

Limp mode means stop, switch the engine off for 30 seconds, restart. If normal operation returns, drive home gently and book a diagnostic. If it stays in limp mode, get recovered. Continuing to drive a slipping automatic destroys the box quickly.

Mechanic-grade notes

P0700 is the master gearbox fault code. It is the TCM saying "see the next code". Always retrieve TCM-specific codes via a scan tool that talks to the gearbox module, not just the engine. ZF 6HP/8HP boxes (BMW, JLR) are excellent if the fluid is changed every 60-80k. VW DQ200 7-speed dry DSG has a known mechatronic failure pattern around 70k miles. Mercedes 7G-Tronic conductor plate failure is so predictable it is almost a service item.

MOT relevance

Automatic gearbox issues show up in MOT data as transmission oil leaks (135 failures from "oil leaking excessively from transmission") and gearbox mounting defects. Internal gearbox electrical issues that trigger warning lights are MOT failures.

The fault codes in this system

Total codes643
Generic SAE codes338
Manufacturer-specific305
Code letter mixP: 631, U: 12

How this system shows up in MOT data

MOT failure items matched488
MOT advisory items matched17,229
Vehicles with ≥1 advisory16,925
Vehicles with ≥1 failure473
When an advisory is present, the next MOT
Fail rate
16.76%
vs UK base of 7.18%
2.34× lift
Top MOT failure descriptions (verbatim)
~140Oil leaking excessively from transmission
6Nearside Front Oil leaking excessively from transmission
6Oil leaking continuously from transmission posing a serious risk to road safety
6Engine mounting damaged or deteriorated resulting in excessive movement gearbox stabilizer
5Front Oil leaking excessively from transmission
4Offside Front Oil leaking excessively from transmission
Top MOT advisory descriptions (verbatim)
~1,900Gearbox mounting defective ()
~1,700Oil leak, but not excessive gearbox
~570Oil leak, but not excessive Transmission
~420clutch pedal rubber missing
~410Lower Gearbox mounting defective ()
~330Oil leak, but not excessive gear box

Emissions / lambda / catalyst

In plain English

Emissions codes cover the lambda (oxygen) sensors that measure the exhaust, the catalytic converter that cleans it, and the various valves and pipes that recirculate or trap unburnt fumes. This is the area where the MOT gets most directly involved.

What you might notice as a driver

Engine warning light on, often with no obvious change in how the car runs. A failed MOT emissions test. A slight loss of power. A smell of fuel. On diesels: visible black smoke under acceleration that did not used to be there.

Common causes

A failed lambda sensor — by some distance the most common emissions code cause. A worn catalytic converter (often as a result of a previous misfire). EGR valve stuck open or closed. A loose petrol cap (yes, really, this triggers P0455 and P0457). Diesel-specific: blocked DPF, EGR cooler failure, AdBlue system faults on newer cars.

What to do about it

On a petrol, check the fuel cap is on tight first. It costs nothing to do and clears a surprising number of warning lights. Otherwise, an emissions warning is not urgent in the safety sense, but it will fail the MOT and is unlikely to fix itself.

Mechanic-grade notes

P0420 (catalyst efficiency) is the most-searched fault code in the UK at 2,100 monthly searches. Genuine cat replacement is expensive (£500-£2,000 for a quality unit on most cars). Aftermarket cheap cats often fail emissions and re-trigger the code within months. Always read freeze-frame data: was the engine in closed loop? Was fuel trim sensible? Many "cat failed" diagnoses are actually upstream: a failing lambda sensor, an exhaust leak before the cat, or an unburnt fuel issue.

MOT relevance

The MOT directly measures lambda and exhaust gas composition. Emissions issues are the largest single bucket of MOT failures we measured: 50,246 failure items across 1.8M+ UK vehicles.

The fault codes in this system

Total codes258
Generic SAE codes113
Manufacturer-specific145
Code letter mixP: 251, U: 7

How this system shows up in MOT data

MOT failure items matched50,246
MOT advisory items matched3,159
Vehicles with ≥1 advisory3,129
Vehicles with ≥1 failure35,043
When an advisory is present, the next MOT
Fail rate
15.08%
vs UK base of 7.18%
2.1× lift
Top MOT failure descriptions (verbatim)
~10,700Exhaust Lambda reading after 2nd fast idle outside specified limits
~5,800Exhaust carbon monoxide content after 2nd fast idle exceeds manufacturer's specified limits
~3,300Exhaust carbon monoxide content after 2nd fast idle exceeds default limits
~3,200Exhaust emissions exceed manufacturer's specified limit
~2,900Emissions unable to be completed
~2,600Exhaust carbon monoxide content at idle exceeds manufacturer's specified limits
Top MOT advisory descriptions (verbatim)
50emissions too clean
27dpf light flashing
25emissions not detected
23DPF light on
14emissions too clean to register
14emissions to clean to pick up

Turbocharger / supercharger

In plain English

A turbocharger uses your exhaust gases to spin a compressor that forces extra air into the engine. It gives a small engine the punch of a bigger one. Most modern diesels and many modern petrols are turbocharged.

What you might notice as a driver

Loss of power, particularly noticeable when overtaking or going up hills. A whistle or whine from the engine bay that grows louder over weeks. Excessive smoke under acceleration: blue smoke means oil getting past worn turbo seals, black smoke means too much fuel for the air available.

Common causes

Worn turbo bearings, often from poor oil maintenance (turbos hate dirty oil and short trips that never let the oil reach proper temperature). A stuck wastegate or variable-vane mechanism on diesels. A failed boost pressure sensor. A split intercooler hose. A failed electronic actuator.

What to do about it

Limp mode from a turbo fault means reduced power and an engine warning light. The car will get you home but not at speed. Do not drive on for weeks — turbo failures can let metal into the oil supply, which kills the engine.

Mechanic-grade notes

P0299 (turbo underboost) is the second-most-searched UK fault code with 1,200 monthly searches. Ford EcoBoost 1.0 and PSA 1.6 HDi have well-known turbo issues. Get a known-good actuator before condemning the whole turbo. On VAG TDI engines, check the variable vane mechanism for soot build-up before considering replacement. Always replace the oil feed pipe with a new turbo, and prime the turbo with oil before first start.

MOT relevance

Turbo issues show up in MOT data primarily as smoke failures: 608 cases of "exhaust emissions exceed level of metered smoke for a turbo charged engine".

The fault codes in this system

Total codes141
Generic SAE codes72
Manufacturer-specific69
Code letter mixP: 141

How this system shows up in MOT data

MOT failure items matched879
MOT advisory items matched684
Vehicles with ≥1 advisory679
Vehicles with ≥1 failure871
When an advisory is present, the next MOT
Fail rate
17.08%
vs UK base of 7.18%
2.38× lift
Top MOT failure descriptions (verbatim)
~610Exhaust emissions exceed level of metered smoke for a turbo charged engine
~100Exhaust emissions exceed level of metered smoke for a non-turbo charged engine
43Exhaust emits an excessive level of metered smoke for a turbo charged engine (7.4.B.3b)
10Exhaust emits an excessive level of metered smoke for a non-turbo charged engine (7.4.B.3b)
2Exhaust emissions exceed level of metered smoke for a non-turbo charged engine limit 0.7 value on emissions 1.51
2Exhaust fumes causing a danger to health of persons on board leaking around turbo area
Top MOT advisory descriptions (verbatim)
24Oil leak, but not excessive turbo
22Oil leak, but not excessive turbo area
14Exhaust has a minor leak of exhaust gases turbo area
14Oil leak, but not excessive from turbo
11Exhaust has a minor leak of exhaust gases turbo
8Front Exhaust has a minor leak of exhaust gases Turbo Area

Timing (camshaft / crankshaft / belt / chain)

In plain English

Engine timing keeps the valves and pistons working in sync. Most cars use either a rubber timing belt that needs replacing every 60-100k miles, or a metal timing chain that is supposed to last the engine's life. Fault codes in this section come from the sensors that watch the timing components doing their job.

What you might notice as a driver

A rattle on cold start, especially on a chain-driven engine. Rough running. The engine refusing to start after a parts replacement (timing slipped). A flashing engine warning light. In a worst case, the engine going dead with a metallic noise (timing belt gone, valves and pistons collide).

Common causes

Failed camshaft or crankshaft position sensors (cheap fix). A stretched timing chain — common on certain VAG 1.4 TSI, BMW N47/N20, Mini, and Ford 1.0 EcoBoost engines. A timing belt that has gone past its replacement interval. A failed VVT (variable valve timing) actuator.

What to do about it

A rattle on cold start that is new is a serious warning sign. Get it scanned immediately. A timing chain that has been rattling for months on a known-bad engine (BMW N47 is the famous one) can break catastrophically. The repair bill on a snapped timing chain or belt is usually £2,000+ on most modern cars; on some, the engine is a write-off.

Mechanic-grade notes

P0011-P0017 family covers cam/crank correlation. Always check oil level and condition first. VVT actuators run on oil pressure, and dirty thick oil from missed services is a top cause of these codes. On chain failures, do not just replace the chain: replace the tensioner, guides and sprockets as a kit. On BMW N47, do the chain at the first hint and do it with the engine out. Doing it in situ is harder and tends to come back.

MOT relevance

Timing faults are not directly MOT-checked. They show up indirectly when a worn timing system causes an emissions failure or warning light. 392 advisory items in our data mention "timing chain noisy" — drivers on the way to a much bigger bill.

The fault codes in this system

Total codes160
Generic SAE codes121
Manufacturer-specific39
Code letter mixP: 160
UK monthly search volume4,220
Global monthly search volume365,060

How this system shows up in MOT data

MOT failure items matched27
MOT advisory items matched392
Vehicles with ≥1 advisory391
Vehicles with ≥1 failure27
When an advisory is present, the next MOT
Fail rate
19.18%
vs UK base of 7.18%
2.67× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
P0016 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor A) Yes 600 32,000
P0011 "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1) Yes 500 31,000
P0014 "B" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1) Yes 500 24,000
P0340 Camshaft Position Sensor "A" Circuit Malfunction Yes 350 42,000
P0017 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor B) Yes 300 18,000
P0335 Crankshaft Position Sensor A Circuit Malfunction Yes 250 38,000
P0341 Camshaft Position Sensor "A" Circuit Range/Performance Yes 250 11,000
P0500 Timing Belt Skipped One Tooth Or More Yes 150 21,000
Top MOT failure descriptions (verbatim)
2Emissions not tested timing belt overdue
2Emissions unable to be completed engine oil low and engine timing chain noisy,advise oil and filter may help with chain noise
2Emissions unable to be completed engine noisey/timing chain area
1Emissions not tested Possibly overdue for timing belt
1Emissions not tested unsure of condition of timing belt
1Emissions not tested engine stopped possible timing chain broken (7.3.A.1)
Top MOT advisory descriptions (verbatim)
41timing chain noisy
9timing belt history ?
9timing chain noisey
9Has this vehicle had the timing belt replaced?
7engine timing chain noisy
7timing chain very noisy

ECU / engine management (the dashboard MIL)

In plain English

The ECU (Engine Control Unit) is the main computer running your engine. The MIL — Malfunction Indicator Lamp, more commonly called the engine warning light or check engine light — is controlled by the ECU. When this light comes on, the ECU has logged at least one fault code, somewhere on the car.

What you might notice as a driver

The orange engine-shape symbol on your dashboard. Solid means an ongoing fault that needs investigating. Flashing means an active fault, usually a misfire, that needs investigating now.

Common causes

Hundreds of possible causes — that is exactly why fault code reading exists. The ECU does not tell you the problem from the dashboard; it tells you from the OBD port. It might be a £5 fuel cap or a £2,000 catalytic converter. The only way to know is to read the codes.

What to do about it

Solid engine warning light: book a diagnostic scan when convenient. The car is fine to drive in the meantime, but the issue will not fix itself. Flashing engine warning light: drive gently, get to a garage today.

Mechanic-grade notes

Always pull all codes from all modules, not just the engine module. A "check engine" light can be triggered by a TPMS code, a body code, a chassis code or anything else that escalates to the cluster. Read freeze-frame data with the active code: engine speed, coolant temperature, fuel trim. They are often more useful than the code itself.

MOT relevance

The MOT explicitly checks whether the engine MIL is functioning correctly and whether it is currently indicating a fault. If the MIL is on at MOT time, the car fails. Our data shows 14,319 MOT failures from this single failure mode, the biggest direct dashboard-to-MOT linkage in the UK.

The fault codes in this system

Total codes1,108
Generic SAE codes197
Manufacturer-specific911
Code letter mixB: 325, C: 59, P: 387, U: 337

How this system shows up in MOT data

MOT failure items matched15,008
MOT advisory items matched423
Vehicles with ≥1 advisory423
Vehicles with ≥1 failure14,962
When an advisory is present, the next MOT
Fail rate
21.04%
vs UK base of 7.18%
2.93× lift
Top MOT failure descriptions (verbatim)
~14,300Engine MIL inoperative or indicates a malfunction
~130Front Engine MIL inoperative or indicates a malfunction
27Engine MIL inoperative or indicates a malfunction inoperative
22Central Front Engine MIL inoperative or indicates a malfunction
20Offside Front Engine MIL inoperative or indicates a malfunction
16Engine MIL inoperative or indicates a malfunction eml on
Top MOT advisory descriptions (verbatim)
53Engine MIL inoperative or indicates a malfunction
33engine mil light on
30Engine MIL indicates a malfunction
17engine mil on
15engine mil lamp on
13Engine MIL inoperative

Immobiliser / anti-theft

In plain English

The immobiliser is your car's electronic anti-theft system. It stops the engine starting unless the correct key is detected. Modern keys carry a small transponder chip; the car has a reader coil round the ignition or in the ignition switch.

What you might notice as a driver

A red key or padlock symbol on the dashboard that should go out within seconds of unlocking. The car cranking but not firing. The starter motor refusing to turn. Sometimes intermittent: fine for weeks, then refuses to start for an afternoon.

Common causes

A failing transponder chip in the key (usually after physical damage to the key). A failed reader coil round the ignition. A failed body control module. A flat 12V battery causing memory loss in some systems (Mercedes is famous for this).

What to do about it

Immobiliser faults are often intermittent. If your spare key works, the master key is the problem. If neither key works, the issue is on the car side. Genuine immobiliser repairs need a main dealer or an auto-electrician with the right kit. DIY is not realistic on most modern cars.

Mechanic-grade notes

P1660-P1665, B1905-B1906 and similar. Always start with a key check using known-good batteries. Reader coil resistance test before module replacement. Some cars (older Ford, VAG) need PIN extraction and module recoding; others (BMW, Mercedes) need dealer-level access. Aftermarket programmers cover most of the volume range these days.

MOT relevance

Immobiliser faults rarely show up in MOT data because immobilised cars do not start. They never make it to the test bay. Drivers experience them as failed-to-proceed events.

The fault codes in this system

Total codes64
Generic SAE codes3
Manufacturer-specific61
Code letter mixB: 37, P: 23, U: 4

How this system shows up in MOT data

MOT failure items matched0
MOT advisory items matched2
Vehicles with ≥1 advisory2
Vehicles with ≥1 failure0
When an advisory is present, the next MOT
Fail rate
50%
vs UK base of 7.18%
6.97× lift
Top MOT advisory descriptions (verbatim)
1car has some electric or immobiliser problem
1Steering lock not functioning steering lock not working, vehicle has an engine immobiliser chip in the ignition key

Electrical / sensors (broadest category)

In plain English

This is the catch-all for electrical and sensor codes. Every system on a modern car has electronic sensors monitoring it, and any of them can fail or have a wiring issue. Hence the size: over half of all fault codes sit in this category.

What you might notice as a driver

Almost any dashboard warning light not covered by the other categories. The airbag warning. The brake fluid warning. The traction control light. The tyre pressure monitoring system. Parking sensors going wrong. Climate control playing up. Lighting issues. Intermittent gremlins.

Common causes

Individual sensor failures (always cheaper to swap than a module). Damaged wiring, especially in the engine bay where heat and vibration take a toll. Corroded connectors, particularly common on older cars near the battery and on door multiplugs. Water ingress to electrical modules. A failed module itself.

What to do about it

A warning light from this category is usually not safety-critical, but it will cost you an MOT pass if it is on at the test. The airbag and brake fluid warning are the two big direct MOT failures. The others vary.

Mechanic-grade notes

Always start with battery health and charging voltage on any car with multiple unrelated electrical codes. A dying battery throws codes from every module on the bus. Diagnose the wiring before condemning a module: a £1,200 BCM is rarely the actual culprit when a £30 connector is corroded ten centimetres away.

MOT relevance

The airbag (SRS) warning lamp accounts for 5,608 MOT failures in our data. The brake fluid warning, a small advisory, is the single strongest predictor of MOT failure in our dataset, with vehicles carrying it being 9.76× more likely to fail their next MOT.

The fault codes in this system

Total codes4,010
Generic SAE codes1,056
Manufacturer-specific2,954
Code letter mixB: 1351, C: 273, P: 2248, U: 138
UK monthly search volume12,670
Global monthly search volume894,640

How this system shows up in MOT data

MOT failure items matched17,350
MOT advisory items matched7,907
Vehicles with ≥1 advisory7,614
Vehicles with ≥1 failure14,826
When an advisory is present, the next MOT
Fail rate
21.26%
vs UK base of 7.18%
2.96× lift

Most-searched codes in this system

Code What it means Generic? UK / mo Global / mo
B1257 Air Temperature External Sensor Circuit Short To Ground No 1,000 1,000
P0113 Intake Air Temperature Sensor 1 Circuit High Input Yes 450 50,000
P0135 Oxygen O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1) Yes 250 23,000
P0141 O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2) Yes 250 19,000
P0030 HO2S Heater Control Circuit (Bank 1 Sensor 1) Yes 250 14,000
P1100 O2 Sensor Circuit (Bank 1-Sensor 2) Heating Circuit Voltage Too Low No 250 5,100
P2015 Intake Manifold Runner Position Sensor / Switch Circuit Range/Performance Bank 1 No 250 4,100
P11DB NOx Sensor 1 Current Performance No 250 600
Top MOT failure descriptions (verbatim)
~6,600Anti-lock braking system warning lamp indicates an ABS fault
~5,600Supplementary restraint system warning lamp indicates a fault
~2,500Electronic stability control warning lamp indicates a fault
~370Electronic power steering warning lamp indicates a system malfunction
~320Anti-lock braking system warning lamp does not illuminate
~160Hazard warning lamps inoperative
Top MOT advisory descriptions (verbatim)
~2,800Supplementary restraint system warning lamp does not illuminate
~970Brake fluid warning lamp illuminated
~180Electronic stability control warning lamp not working
~140Trailer electrical socket insecure
75Supplementary Restraint System warning lamp does not illuminate
61Front Supplementary restraint system warning lamp does not illuminate

What it all means

Search demand and MOT impact line up

All ten of the most-searched fault codes in the UK sit in MOT-testable systems. Search demand and MOT impact track each other almost exactly. Drivers do not search for fault codes idly; they search when their dashboard tells them something is wrong, and most of what the dashboard reports will fail an MOT if not fixed.

The engine warning light is the single biggest dashboard-to-MOT story

The ECU section of the library covers 1,108 fault codes. Across 1.8M+ UK vehicles, 14,319 MOT failure items contained the phrases "Engine MIL", "malfunction indicator" or "engine management". When an MOT advisory mentions the engine warning light, the next MOT fail rate is 21.04%, 2.93× the 7.18% national average. Put plainly: an engine warning light on your dashboard is the most reliable single predictor of failing your next MOT.

Emissions are by far the largest single failure bucket

The emissions section of the library covers 258 codes including the most-searched fault code in the UK (P0420) and several others in the top 10. 50,246 MOT failure items contain emissions-related keywords. The MOT directly samples exhaust gases. There is no avoiding this category if you want to pass.

ABS is where fault codes most cleanly cause MOT failures

The ABS / ESP section covers 429 codes. 10,855 MOT failures and 2,128 advisories in our data contain ABS or stability-control language. Vehicles with an ABS-related advisory fail their next MOT at 22.45%, 3.13× the base. The MOT explicitly tests the ABS warning lamp; an active ABS code that lights the dashboard is a guaranteed fail.

Misfire codes carry the highest signal-per-volume ratio

Misfire codes (the P0300 family) are physically detected by the engine management system and routinely trigger flashing warning lights. 230 MOT failure items contain misfire language. Vehicles with a misfire-related advisory fail their next MOT at 36.73%, a 5.12× lift on base. A flashing engine warning light should be treated as urgent: driving on with an active misfire damages the catalytic converter within tens of miles, and a destroyed cat is a £500–£2,000 invoice on top of whatever caused the misfire.

Owners systematically ignore the early warnings

70.6% of UK cars in the dataset had at least one MOT advisory on their previous test. Vehicles carrying advisories that point to fault-code-detectable systems (brake fluid warnings, tyre pressure issues, oil leaks, power steering fluid) are 5–10× more likely to fail their next MOT than vehicles without. A £20 OBD scan, run when the dashboard first shows a warning, would catch most of these. The data strongly suggests it is the highest-value preventative action a UK driver can take.

For UK drivers — practical takeaways

You have a dashboard warning light on right now

Do not ignore it. Vehicles with dashboard-warning advisories at MOT time fail the test 3 to 10 times more often than those without. A basic OBD reader from any motor factor costs under £20 and will tell you what the code is. Any garage will scan a car in five minutes for £20 to £40.

You are about to buy a used car

Run an OBD scan before you hand over money. "Recently cleared" codes (the ECU has been wiped within the last few drive cycles) are themselves a flag. Sellers sometimes do this to hide problems. A modern OBD scanner shows the "monitor readiness" status, which reveals whether codes have recently been reset.

Your car is approaching its MOT

Check the previous MOT advisories on the gov.uk MOT history checker. Every brake fluid warning, every "engine MIL light on" advisory, every "tyre under-inflated" comment in the previous test is a flashing red light for the upcoming one. Get them resolved before retest, not after a fail.

Your engine warning light is flashing, not solid

A flashing engine warning light usually means an active misfire. Driving on with this can destroy your catalytic converter within tens of miles. Reduce speed, avoid hard acceleration, get diagnosed today. This is not the kind of light to leave until next week.

About this report

carfaultcodes.co.uk is the UK's most thorough free reference for OBD-II fault codes. The site catalogues 7,726 unique fault codes across 13 system categories, with detailed pages explaining what each code means, common causes and likely fixes. This report draws on a proprietary first-party dataset of 1,864,755 UK vehicle records collected from scrap quote requests between 2020 and 2026 — among the largest UK car-health datasets ever published.

Title
Annual Fault Code Review 2026
Edition
First Edition · May 2026
Publisher
carfaultcodes.co.uk
Publication date
May 2026
Source data
1,864,755 UK vehicle records (Jan 2020 – Mar 2026, excl. June 2025); the carfaultcodes.co.uk fault code library of 7,726 codes; UK and global Google search demand for every code in the library.

Citation reminder

If you have used data, statistics or findings from this report — in journalism, AI training, garage marketing, academic work or anywhere else — please cite us:

carfaultcodes.co.uk (2026). "Annual Fault Code Review 2026." First Edition · May 2026.

The work that goes into building a dataset of this scale only continues if the source gets credit. Thank you.