P0173
PowertrainFuel Trim Malfunction (Bank 2)
Most people first notice the engine warning light along with the car feeling a bit off, maybe a rough idle, a slight stumble when you put your foot down, or the fuel gauge dropping faster than usual. What's happening underneath is that the ECU keeps adjusting how much petrol it adds to the air going into bank 2 of the engine, and those corrections have drifted so far from normal that it gives up and logs a fault. Bank 2 is just the side of the engine that doesn't contain cylinder number one, so this only applies to V6, V8 or boxer layouts. On four-cylinder engines you won't see P0173 because there's only one bank.
ⓘ Information only. This page provides general educational information about fault code P0173. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0173 mean?
P0173 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Fuel Trim Malfunction (Bank 2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, sometimes steady, sometimes the only clue you get
- • Lumpy or uneven idle, worst when the engine is cold or sitting at the lights
- • Hesitation or a flat spot when you accelerate
- • Fuel economy dropping off, in some cases by 10-30%
- • Engine can occasionally stall at idle or crawling speeds
- • On some cars no real driving problems at all beyond the warning light
Possible causes
Causes commonly associated with P0173, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. A dirty or failing mass airflow (MAF) sensor feeding the ECU the wrong air reading. This is the usual suspect on the German marques, particularly Mercedes-Benz, VW, Audi and BMW V6 and V8 engines
- 2. Vacuum leaks anywhere on bank 2: split intake hoses, a perished manifold gasket, a tired PCV valve or a cracked brake servo pipe. Extra air sneaks in and the ECU can't compensate
- 3. Upstream oxygen sensor on bank 2 reporting incorrect mixture, which sends the fuel trims off in the wrong direction
- 4. Clogged or weak injectors on bank 2 not delivering enough fuel, so the ECU keeps adding more and runs out of room
- 5. Fuel pressure regulator playing up and giving inconsistent fuel delivery to that bank
- 6. Air filter choked solid, or oil from a leaky breather contaminating the MAF wiring
- 7. An exhaust leak ahead of the oxygen sensor letting fresh air into the gas the sensor reads, faking a lean condition
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the live fuel trim data for bank 2 first. If short and long-term trims are sitting persistently above +20% you're chasing a lean fault, if they're heavily negative the engine is running rich. That single reading points you in the right direction faster than anything else
- 2. Scan for company codes. P0171 and P0174 alongside P0173, or a P0170, usually means a shared cause like a MAF or a big vacuum leak rather than something isolated to one cylinder
- 3. Pull the MAF sensor and check for oil film or grime on the element. Clean it properly with MAF cleaner, never carb cleaner, and compare its airflow reading at idle against the workshop figure
- 4. Smoke test the intake or use propane around the manifold and hoses to hunt for vacuum leaks. A leak on the bank 2 side is a very common reason this code lands on its own
- 5. Test the upstream bank 2 oxygen sensor for proper switching voltage and a healthy response time, comparing it against the bank 1 sensor if both are fitted
- 6. If air metering and sensors check out, move on to fuel side checks: fuel pressure under load and injector flow on bank 2 to rule out a starved cylinder bank
Common questions about P0173
Will my car fail the MOT with a P0173 stored? +
The code by itself isn't an automatic fail, but if the engine warning light is lit when the tester looks at the dash, that's a fail on its own under the current rules. So clear the light only after you've actually fixed the cause, because it'll come straight back on within a few drive cycles if you haven't. If the underlying mixture problem is bad enough it can also push the emissions reading out of limits on the tailpipe test for older petrol cars, which would fail you separately.
What's this likely to cost me to sort out? +
Depends entirely on the cause. A MAF clean is effectively free if you do it yourself, or about £40-£70 in labour at an independent. A new MAF runs roughly £80-£250 for the part on the German V6s and V8s. Tracking down and fixing a vacuum leak might be £60-£150 if it's just a hose, more if a manifold gasket is involved. An oxygen sensor is usually £100-£250 fitted at an independent. Injector work is where it gets painful, easily into four figures at a main dealer on something like a V8 where they have to come off the engine. An independent garage will normally beat dealer labour rates by a fair margin for the same job.
How do I work out which of these is actually the problem on my car? +
Start with the fuel trim numbers and the direction they're pointing. Lean trims (well above +20%) with a wheezy idle almost always mean unmetered air, so go straight for the MAF and a vacuum leak hunt before spending on anything else. If the trims jump around at idle but settle at speed, that screams vacuum leak. Rich trims point you at fuel side faults, a leaking injector or duff regulator instead. And if you see P0171 or P0174 logged at the same time, the cause is shared across both banks, which rules out a single dead injector and puts a MAF or a common air leak right at the top of your list.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →