How to read fault codes
Everything you need to know, from plugging in a reader to understanding what the code actually means.
Step by step
- 1
Get an OBD-II reader
You need a code reader to retrieve fault codes. Basic readers cost from £15. Bluetooth ELM327 dongles pair with free smartphone apps. If you don't want to buy one, many garages and fast-fit centres (Halfords, Kwik Fit) will read codes for free.
- 2
Find the OBD port
It's almost always under the dashboard on the driver's side, within arm's reach of the steering wheel. It's a 16-pin trapezoid socket. On some models it has a small cover.
- 3
Plug in with the ignition off
Insert the reader into the OBD port. Then turn the ignition to position II (accessories on, engine off) or start the engine, your reader's instructions will specify which.
- 4
Read the codes
Select 'Read codes' on the reader. It will display any stored or pending fault codes, usually formatted as P0300, C0021, B1001, or similar.
- 5
Look up the code here
Type the code into the search at the top of this page or on the homepage. We'll show you exactly what it means, how serious it is, and what to do next.
- 6
Fix first, then clear
Once the fault is repaired, use the reader's 'Clear codes' function to reset the warning light. Don't clear without fixing, the code will return.
Common questions
What is OBD and where is the port?
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OBD stands for On-Board Diagnostics. All petrol cars made after 2001 and diesel cars made after 2004 sold in the UK are required to have an OBD-II port. It's usually under the dashboard on the driver's side, within 60cm of the steering wheel. It looks like a 16-pin trapezoid socket.
What code reader do I need?
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For basic fault code reading, any generic OBD-II reader works, they start from around £15 online. Bluetooth dongles (ELM327-compatible) pair with a smartphone app. For manufacturer-specific codes and live data, you need a more capable reader, garages use tools like Autel, Launch, or Snap-on.
Can I clear the fault code myself?
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Yes, most code readers have a 'clear codes' function. This turns off the warning light and resets the code. However, it does not fix the underlying problem. The code will return once the car's systems run through their checks again, usually within a few drive cycles.
What's the difference between a generic and manufacturer-specific code?
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Generic OBD-II codes (P0xxx, C0xxx, B0xxx, U0xxx) are standardised across all makes and models, P0300 means the same thing in a Ford as it does in a BMW. Manufacturer-specific codes (P1xxx, P2xxx, C1xxx, B1xxx etc.) are defined by the vehicle manufacturer and can vary between brands for the same code number.
My warning light came on then went off, do I still need to check it?
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Yes. Intermittent faults still store a code in the car's memory even if the light clears itself. These are called 'pending' or 'stored' codes. An OBD reader will find them. Ignoring an intermittent fault often means it returns as a permanent one later.
Is it safe to drive with a warning light on?
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It depends entirely on the code. A minor emissions code or a sensor fault with no drivability impact is usually safe to drive on for a short period. An oil pressure warning, coolant temperature alarm, or any red warning light should be treated as stop immediately. When in doubt, stop and get it checked.