OBD2 Codes
Car diagnostic codes explained for DIY mechanics.
Understand what your check engine light means. Get causes, symptoms, severity, and first steps to diagnose common OBD2 error codes.
OBD2 Codes
P0171 – System Too Lean (Bank 1)
Air/fuel mixture too lean on bank 1; commonly vacuum leaks or a dirty MAF
OBD2 Codes
P0174 – System Too Lean (Bank 2)
Mirror to P0171 on bank 2; usually shares the same root cause.
OBD2 Codes
P0300 – Random Misfire Detected
Multiple cylinders misfiring; can be spark plugs coils or fuel delivery.
OBD2 Codes
P0301 – Cylinder 1 Misfire Detected
Cylinder 1 is misfiring; check spark plug coil and injector.
OBD2 Codes
P0302 – Cylinder 2 Misfire Detected
Cylinder 2 is misfiring; check spark plug coil and injector.
OBD2 Codes
P0303 – Cylinder 3 Misfire Detected
Cylinder 3 is misfiring; check spark plug coil and injector.
OBD2 Codes
P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
Catalytic converter efficiency is below threshold; often caused by upstream issues.
OBD2 Codes
P0430 – Catalyst System Efficiency Below Threshold (Bank 2)
Catalytic converter efficiency is below threshold on bank 2.
OBD2 Codes
P0442 – Evaporative Emission Control System Leak Detected (Small Leak)
Small leak detected in EVAP system; check gas cap first.
OBD2 Codes
P0445 – Evaporative Emission Control System Purge Control Valve Circuit Shorted
EVAP purge valve circuit issue; check wiring and valve.
OBD2 Codes
P0455 – Evaporative Emission Control System Leak Detected (Large Leak)
Large leak detected in EVAP system; check hoses and connections.
OBD2 Codes
P0172 – System Too Rich (Bank 1)
Air/fuel mixture too rich on bank 1; check fuel pressure and injectors.
OBD2 Codes
P0175 – System Too Rich (Bank 2)
Air/fuel mixture too rich on bank 2.
OBD2 Codes
P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Engine not reaching proper operating temperature; thermostat likely stuck open.
OBD2 Codes
P0117 – Engine Coolant Temperature Sensor Circuit Low Input
Coolant temperature sensor reading too low; check sensor and wiring.