OBD-II Diagnostic Trouble Code Library
Free reference for the most-searched OBD-II trouble codes — what each code means, common symptoms, possible causes, what a technician inspects, and the mistakes to avoid. Codes identify detected conditions, not failed parts: proper diagnosis always requires testing.
Misfire
5 CODESRandom/Multiple Cylinder Misfire Detected
The engine control module has detected misfires occurring across multiple cylinders or in a random pattern, rather than one identifiable cylinder.
Cylinder 1 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 1.
Cylinder 2 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 2.
Cylinder 3 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 3.
Cylinder 4 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 4.
Fuel System
2 CODESSystem Too Lean (Bank 1)
The engine control module has detected that the air-fuel mixture on bank 1 is leaner than the calibrated target — more air (or less fuel) than expected.
System Too Rich (Bank 1)
The engine control module has detected that the air-fuel mixture on bank 1 is richer than the calibrated target — more fuel (or less air) than expected.
Emissions
3 CODESCatalyst System Efficiency Below Threshold (Bank 1)
The engine control module has determined that the catalytic converter on bank 1 is not cleaning exhaust efficiently, based on the difference between upstream and downstream oxygen sensor readings.
Catalyst System Efficiency Below Threshold (Bank 2)
The engine control module has determined that the catalytic converter on bank 2 is not cleaning exhaust efficiently, based on upstream and downstream oxygen sensor readings.
Exhaust Gas Recirculation Flow Insufficient
The engine control module has detected that the EGR system is not flowing enough recirculated exhaust gas when commanded.
EVAP
2 CODESEvaporative Emission Control System Leak Detected (Small Leak)
The EVAP system self-test has detected a small leak in the sealed system that captures and stores fuel vapor from the fuel tank.
Evaporative Emission Control System Leak Detected (Large Leak)
The EVAP system self-test has detected a large leak (sometimes called a gross leak) in the sealed fuel vapor containment system.
Cooling System
1 CODESensors
2 CODESO2 Sensor Circuit (Bank 1, Sensor 1)
The engine control module has detected a circuit or signal fault with the upstream (pre-catalytic converter) oxygen sensor on bank 1.
O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)
The engine control module has detected that the upstream oxygen sensor on bank 1 is switching between rich and lean more slowly than the calibrated response time.
How to use this library
Each page states what the code means, the symptoms and causes commonly associated with it, and what a technician inspects — but a trouble code identifies a condition the computer detected, never a specific failed component. Where behavior varies by manufacturer, the pages say so. Start with the OBD-II introduction if you're new to diagnostics.
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