OBD fault codes explained

What an OBD fault code is

An OBD fault code is a message from the on-board computer, for example P0300. You read it with an OBD reader plugged into the diagnostic socket of the car. Most cars from the past twenty years have one.

A code points to a system or a measurement, not automatically to the faulty part. Do not replace parts based on a code alone.

How a code is built

First letter: P is powertrain (engine and gearbox), B is body, C is chassis, U is communication between control units.

First digit: 0 is a generic code that means the same for every make, 1 is a code defined by the manufacturer.

Second digit of P codes: the subsystem, for example 1 and 2 for fuel and air, 3 for ignition and misfires, 4 for emission control, 5 for idle and speed control.

The last two digits identify the exact fault.

Common generic codes

P0300: random or multiple misfires detected. P0301 to P0306: misfire in cylinder 1 to 6.

P0171 and P0174: fuel mixture too lean (too much air or too little fuel). P0172 and P0175: mixture too rich.

P0420 and P0430: catalytic converter efficiency below threshold.

P0442: small leak in the evaporative emission system. P0455: large leak, sometimes a tank cap not closed properly.

P0128: coolant temperature below thermostat regulating temperature, often a thermostat stuck open.

P0101: mass air flow sensor reading out of range. P0335: fault in the crankshaft position sensor circuit.

These descriptions are general meanings. Causes and repairs differ by car.

What to do with a code

1. Write down the full code and whether it is 'stored' or 'pending'.
2. Look the code up for your make and model; this is necessary for a code starting with 1.
3. Check the simple causes first, such as the tank cap, wiring and filters.
4. Have a garage find the cause if the code returns or a light flashes.

If you clear a code without fixing the problem, it usually returns. Clearing also resets the car's readiness tests, which can affect an inspection.

This is a draft and not professional advice.