Look up any OBD2 / EOBD fault code with plain-English explanations and likely causes.
What an OBD2 fault code actually means
A fault code is not a diagnosis, it is a clue. P0420 does not mean your catalytic converter is dead — it means the ECU has decided the downstream oxygen sensor readings are not what it expects. That can be the cat, a lazy sensor, an exhaust leak upstream of the sensor, or an engine running rich for an entirely separate reason.
Codes are made of a letter and four digits. P is powertrain, B is body, C is chassis and U is network communication. The first digit tells you whether it is a generic code every manufacturer shares (0) or a manufacturer-specific one (1).
How to use the lookup properly
Enter your code above for the plain-English meaning, the symptoms that go with it, the causes ranked by how often they turn out to be the culprit, and realistic UK repair costs.
If you have several codes stored, work on the lowest-numbered and most fundamental one first. A misfire code alongside a lean code is usually one fault causing both, and clearing the underlying cause clears the pair.
Should you clear the code?
Write it down first. Once you clear it you lose the freeze-frame data — the snapshot of engine speed, load and temperature at the moment the fault was logged — and that data is often what pins down an intermittent fault.
Clearing also resets the emissions readiness monitors, which means the car will not pass an MOT emissions check until it has completed a full drive cycle. Never clear codes the morning of an MOT.
When to stop reading and book it in
A flashing engine management light means an active misfire dumping raw fuel into the catalytic converter — stop driving and get it recovered rather than risk a cat that costs several hundred pounds.
If the code involves the brakes, airbags or steering, that is a safety system, not a project. Everything else is fair game to investigate yourself, and the guides here will tell you what to check and in what order.
Frequently asked questions
Is the fault code lookup free?
Yes. The meaning, symptoms, common causes and cost bands for every code are free to read.
Will clearing a fault code fix the problem?
No. It only turns the light off. If the fault is still present the code returns, usually within a few dozen miles.
Do I need an expensive scanner?
A basic OBD2 reader is enough to pull generic P0 codes on any car built since 2001 petrol or 2004 diesel. Manufacturer-specific codes in ABS, airbag or body modules need a better tool.
Can a fault code fail an MOT?
The code itself does not, but an illuminated engine management light is an automatic MOT failure, as is an emissions readiness state that has not completed.