OBD-II diagnostic trouble codes
A diagnostic trouble code is a five-character identifier the vehicle stores when a monitored system reports a fault. The first letter names the system family, the second character says whether the definition is standardised across all manufacturers or specific to one, and the remaining three narrow it to a subsystem and a specific fault.
How to read a code
- P
- System family — powertrain
- 0
- Generic, standardised across manufacturers
- 4
- Subsystem — auxiliary emission controls
- 20
- Specific fault index
Worked example: P0420
Powertrain — P codes (10)
P0101Mass or Volume Air Flow Circuit Range/PerformanceP0128Coolant Thermostat Below Regulating TemperatureP0171System Too Lean (Bank 1)P0174System Too Lean (Bank 2)P0300Random/Multiple Cylinder Misfire DetectedP0301Cylinder 1 Misfire DetectedP0401Exhaust Gas Recirculation Flow InsufficientP0420Catalyst System Efficiency Below Threshold (Bank 1)P0430Catalyst System Efficiency Below Threshold (Bank 2)P0455Evaporative Emission System Leak Detected (Large Leak)
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)