Common codes · Updated November 2, 2026
Twelve trouble codes, in plain English.
What each code means, and what is usually worth checking first. A code is a clue, not a verdict.
| Code | What it means | System | Check first |
|---|---|---|---|
| P0300 | Random or multiple cylinder misfire detected | Ignition | Spark plugs, ignition coils, fuel delivery. If the light flashes, stop and get help. |
| P0301 | Cylinder 1 misfire detected (P0302 to P0306 are the other cylinders) | Ignition | The plug and coil on that cylinder first; swapping parts between cylinders can show whether the fault follows the part. |
| P0420 | Catalyst system efficiency below threshold (bank 1) | Emissions | Oxygen sensors and exhaust leaks before the converter; a failing converter is one possible cause. |
| P0171 | System too lean (bank 1) | Fuel & air | Vacuum leaks, a dirty mass airflow sensor, low fuel pressure. |
| P0172 | System too rich (bank 1) | Fuel & air | Dirty air filter, leaking injector, sensor faults. |
| P0174 | System too lean (bank 2) | Fuel & air | Same checks as P0171, on engines with two banks of cylinders. |
| P0101 | Mass airflow sensor circuit range or performance problem | Fuel & air | A dirty sensor, an air leak after the sensor, wiring. |
| P0442 | Evaporative emission system leak detected (small leak) | Emissions | Start with the gas cap. Hoses and the purge valve are next. |
| P0455 | Evaporative emission system leak detected (large leak) | Emissions | A missing or loose gas cap is a common cause; also hoses and the filler neck. |
| P0128 | Coolant temperature below thermostat regulating temperature | Cooling | A thermostat stuck open, low coolant or a faulty coolant sensor. |
| P0135 | Oxygen sensor heater circuit problem (bank 1, sensor 1) | Emissions | The sensor, its wiring and fuse. |
| P0700 | Transmission control system malfunction | Transmission | A transmission code is stored too; read the transmission-specific code before anything else. |
No codes match.
These are generic OBD2 definitions; manufacturers add their own codes, and the fix depends on the vehicle. Many codes can have several causes. See the Disclaimer. If a light is flashing, stop and get help before looking anything up.