Code Library
Diagnostic trouble codes, decoded
- P: Powertrain
- Engine, transmission, fuel, ignition, and emissions related codes.
- B: Body
- Comfort and safety items such as airbags, seat belts, and climate controls.
- C: Chassis
- Braking, steering, suspension, and traction related systems.
- U: Network
- Communication between control modules over the vehicle data network.
The second character shows whether a code is generic (0) or manufacturer specific (1). The remaining digits point to the system and the specific fault.
Common codes
| Code | Name | System | What it generally means |
|---|---|---|---|
| P0300 | Random or multiple cylinder misfire | Ignition | The engine computer detected misfires that are not limited to one cylinder. Spark, fuel delivery, and air leaks are common areas a technician reviews. |
| P0171 | System too lean, bank 1 | Fuel and air | The engine is getting more air or less fuel than expected on one side. Vacuum leaks and airflow sensor readings are typical starting points. |
| P0420 | Catalyst efficiency below threshold, bank 1 | Emissions | Downstream oxygen sensor readings suggest the catalytic converter is not storing oxygen as expected. |
| P0442 | Small evaporative system leak | EVAP | A small leak was found in the system that captures fuel vapors. A loose or worn fuel cap is one frequently cited cause. |
| P0128 | Coolant temperature below thermostat range | Cooling | The engine took longer than expected to warm up, which often points toward the thermostat or temperature sensor. |
| P0101 | Mass airflow sensor range or performance | Air intake | Airflow readings do not match what the computer expects for current engine conditions. |
| P0455 | Large evaporative system leak | EVAP | A larger leak was detected in the fuel vapor system, such as a missing cap or disconnected hose. |
| P0500 | Vehicle speed sensor malfunction | Transmission | The computer is not receiving an expected vehicle speed signal. |