OBD-II fault codes

Take P0301: P is the powertrain, 0 means the code is generic across all makes under SAE J2012, 3 is the ignition and misfire subsystem, and 01 is the number: misfire in cylinder one. Codes with a one in second place (P1, B1, C1, U1) are defined by the manufacturer, so the same number means different things on different makes - look those up in the documentation for your car.

The 16-pin OBD-II socket sits in the cabin within reach of the driver: most often under the dashboard to the left of the steering column, sometimes behind a cover near the ashtray, the armrest or the fuse box. It is mandatory on petrol cars from 2001 in Europe (EOBD) and from 1996 in the United States.

• Write down the code and the freeze frame before clearing - that data disappears on reset.
• Check the simple things: oil and coolant level, the fuel cap, connectors and wiring at the named sensor.
• Treat the code as a symptom, not a verdict on a part: P0420 is often caused by a worn oxygen sensor or an exhaust leak rather than the catalyst itself.
• Fix power and wiring codes first (P0562, U0100) - they spawn a cascade of unrelated faults.
• Clear the code only after the repair, then drive a cycle to confirm it does not return.

A steady light is a reason to drive calmly to a workshop. A flashing one means active misfires: unburnt fuel reaches the catalyst and can ruin it in a matter of minutes. Reduce speed, take the load off the engine and end the trip as early as you safely can.

The letter is the system: P powertrain, B body, C chassis, U network. The first digit says whether the code is generic under SAE (0) or added by the manufacturer (1). For P0 codes the second digit points to the subsystem. The last two are the number of the specific fault.

If the light is steady and the car drives normally, getting to a workshop is fine. A flashing light means misfires and a threat to the catalyst: reduce load and end the trip.

Clearing removes the record, not the cause. The module repeats its test under its own conditions and logs the fault again. Reset only after a repair.

No. A code describes a symptom. P0171 usually means an air leak or a dirty MAF rather than a failed oxygen sensor. Check wiring, connectors and leaks first.

A snapshot of parameters at the moment the fault was logged: rpm, load, temperature, speed, fuel trims. It shows the conditions in which the fault appears - often more useful than the code itself.