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 (47)

P0010Camshaft Position Actuator Circuit (Bank 1)P0011Camshaft Position Timing Over-Advanced (Bank 1)P0014Camshaft Position Timing Over-Advanced (Bank 1 Exhaust)P0016Crankshaft Position — Camshaft Position Correlation (Bank 1 Sensor A)P0030HO2S Heater Control Circuit (Bank 1 Sensor 1)P0031HO2S Heater Control Circuit Low (Bank 1 Sensor 1)P0100Mass or Volume Air Flow Circuit Malfunction P0101Mass or Volume Air Flow Circuit Range/Performance P0102Mass or Volume Air Flow Circuit Low Input P0110Intake Air Temperature Circuit Malfunction P0113Intake Air Temperature Sensor Circuit High Input P0117Engine Coolant Temperature Circuit Low Input P0118Engine Coolant Temperature Circuit High Input P0120Throttle Pedal Position Sensor A Circuit Malfunction P0121Throttle Position Sensor A Circuit Range/Performance P0128Coolant Thermostat Below Regulating Temperature P0130O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)P0131O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)P0135O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)P0136O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)P0138O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)P0140O2 Sensor Circuit No Activity (Bank 1 Sensor 2)P0141O2 Heater Circuit Malfunction (Bank 1 Sensor 2)P0150O2 Sensor Circuit Malfunction (Bank 2 Sensor 1)P0171System Too Lean (Bank 1)P0172System Too Rich (Bank 1)P0174System Too Lean (Bank 2)P0217Engine Over Temperature Condition P0299Turbocharger/Supercharger Underboost P0300Random/Multiple Cylinder Misfire Detected P0301Cylinder 1 Misfire Detected P0302Cylinder 2 Misfire Detected P0303Cylinder 3 Misfire Detected P0304Cylinder 4 Misfire Detected P0325Knock Sensor 1 Circuit Malfunction (Bank 1)P0335Crankshaft Position Sensor A Circuit Malfunction P0340Camshaft Position Sensor Circuit Malfunction P0401Exhaust Gas Recirculation Flow Insufficient P0420Catalyst System Efficiency Below Threshold (Bank 1)P0430Catalyst System Efficiency Below Threshold (Bank 2)P0442Evaporative Emission System Leak Detected (Small Leak)P0446Evaporative Emission System Vent Control Circuit P0455Evaporative Emission System Leak Detected (Large Leak)P0480Cooling Fan 1 Control Circuit P0507Idle Air Control System RPM Higher Than Expected P0562System Voltage Low P0700Transmission Control System Malfunction

Network — U codes (1)

U0100Lost Communication With ECM/PCM A

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)