How do I read a fault code on my car?

You need a code reader plugged into the diagnostic socket, which on any car built since 2001 is within reach of the driver's seat — usually under the dashboard near the steering column, sometimes behind a small trim panel or in the fusebox. A basic reader will show engine codes; a better one adds the body, chassis and network systems and can show live sensor readings. Many garages and parts shops will read codes for nothing. Reading a code does not clear it, so there is no harm in looking.

Does clearing a fault code fix the problem?

No. Clearing a code erases the record and turns the warning light off, but the computer will set it again the moment it repeats the check that failed. It also does something worse before an MOT: clearing codes resets the on-board readiness monitors, which a tester can see have not completed. On many cars the emissions check cannot finish until they have run, which needs a drive cycle that can take up to a hundred miles. So the car fails on an incomplete emissions test instead of a warning light.

Will a fault code fail my MOT?

Not directly — a tester does not plug a code reader in. What fails the car is the warning light the code turns on. An illuminated engine management lamp is a major defect on petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and it is the largest warning-light defect in the DVSA data, with 371,859 recorded failures. The airbag, ABS, brake, stability control and tyre pressure lamps are all major defects too. So a stored code with no light showing is not an MOT problem; the same code with the lamp on is.

What is the difference between a generic and a manufacturer-specific code?

The second character tells you. A 0, 2 or 3 means the code is defined by the SAE standard, so P0420 means the same thing on a Ford as on a Toyota. A 1 means the manufacturer defined it themselves, so the number is real but the meaning is specific to that make and a generic reader will often show the code without a description. Roughly a fifth of the codes in circulation are manufacturer-specific.

Can I still drive with a warning light on?

It depends on the colour and the fault. A steady amber engine management light usually means you can drive on and get it looked at, but a flashing one means a misfire severe enough to destroy the catalytic converter and you should stop. Red lamps for oil pressure, coolant temperature or the brake system mean stop as soon as it is safe. Each code page here gives a plain verdict on whether the car is safe to keep using.

Car fault codes

Every OBD-II fault code, what it actually means, which warning light it turns on, and whether it will fail your MOT. Type a code, or describe the fault.

Last updated 27 July 2026

Quick answer

A fault code is the engine or body computer’s own record of what it found wrong, read out through the socket under the dashboard. The letter tells you the system: P for engine and emissions, B for body and airbags, C for brakes, steering and tyres, U for the network between them. A code is not a diagnosis — it names the circuit or reading that failed a check, not the part that caused it. This lookup holds 9,533 definitions.

Haven’t got a code, just a light?

You do not need a code reader to start. If you can see the symbol, the dashboard warning light guide covers every one a UK driver is likely to see, whether the car is safe to keep driving, and which of them fail an MOT on their own.

The four code families

The first character is the most useful thing on the code, because it tells you which system reported the fault before you look anything up.

PPowertrain: Engine and emissions. 7,355 codes.
CChassis: Chassis, brakes and steering. 626 codes.
UNetwork: Network and communication. 1,229 codes.
BBody: Body and restraints. 323 codes.

Codes we have written about in full

115 codes have a page of their own, covering how the fault is detected, what causes it in order of likelihood, whether you can keep driving and what it means for the MOT. The rest are defined in the search above.

P codes

B codes

C codes

U codes

Common questions

How do I read a fault code on my car?: You need a code reader plugged into the diagnostic socket, which on any car built since 2001 is within reach of the driver's seat — usually under the dashboard near the steering column, sometimes behind a small trim panel or in the fusebox. A basic reader will show engine codes; a better one adds the body, chassis and network systems and can show live sensor readings. Many garages and parts shops will read codes for nothing. Reading a code does not clear it, so there is no harm in looking.
Does clearing a fault code fix the problem?: No. Clearing a code erases the record and turns the warning light off, but the computer will set it again the moment it repeats the check that failed. It also does something worse before an MOT: clearing codes resets the on-board readiness monitors, which a tester can see have not completed. On many cars the emissions check cannot finish until they have run, which needs a drive cycle that can take up to a hundred miles. So the car fails on an incomplete emissions test instead of a warning light.
Will a fault code fail my MOT?: Not directly — a tester does not plug a code reader in. What fails the car is the warning light the code turns on. An illuminated engine management lamp is a major defect on petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and it is the largest warning-light defect in the DVSA data, with 371,859 recorded failures. The airbag, ABS, brake, stability control and tyre pressure lamps are all major defects too. So a stored code with no light showing is not an MOT problem; the same code with the lamp on is.
What is the difference between a generic and a manufacturer-specific code?: The second character tells you. A 0, 2 or 3 means the code is defined by the SAE standard, so P0420 means the same thing on a Ford as on a Toyota. A 1 means the manufacturer defined it themselves, so the number is real but the meaning is specific to that make and a generic reader will often show the code without a description. Roughly a fifth of the codes in circulation are manufacturer-specific.
Can I still drive with a warning light on?: It depends on the colour and the fault. A steady amber engine management light usually means you can drive on and get it looked at, but a flashing one means a misfire severe enough to destroy the catalytic converter and you should stop. Red lamps for oil pressure, coolant temperature or the brake system mean stop as soon as it is safe. Each code page here gives a plain verdict on whether the car is safe to keep using.

Where these definitions come from

The 9,533 standard definitions come from OBDex (github.com/foerbsnavi/OBDex), data under CC0 1.0, an openly licensed OBD-II dataset. Which dashboard lamp a code illuminates is derived from the code’s own family under SAE J2012 rather than taken from that dataset, because we found its lamp field marked hundreds of body and chassis codes as lighting the engine lamp, which they do not. The MOT consequences come from the DVSA inspection manual and from the MOT test data behind the rest of this site.

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