OBD2 Fault Code Library

Free lookup for thousands of OBD2 trouble codes: filter by family, read short meanings, and open deeper articles. Desktop ELM327 scanner with live data and AI guidance.

OBD2 symptoms

Look up diagnostic trouble codes by what the car is doing: misfire, overheating, rough idle, a check engine light, and other common symptoms.

• Engine misfire
• Engine overheating
• Rough idle
• Check engine light
• Poor fuel economy
• Hard starting
• Engine stalling
• Catalytic converter efficiency
• Oxygen sensor fault
• EVAP leak
• Thermostat not reaching temperature
• Lean air-fuel mixture
• Rich air-fuel mixture
• EGR flow fault
• Engine knock
• Charging system
• Throttle position
• Mass airflow sensor
• Turbo underboost
• Coolant temperature sensor
• Transmission fault

• P0425 — OBD2 code P0425 flags a problem with the catalytic converter efficiency monitoring (catalyst temperature sensor (bank 1, sensor 1)). The car is usually drivable while you plan the repair. Common culprits: aged or contaminated catalytic converter, persistent rich or lean fuel mixture, exhaust leak ahead of the converter.
• P0426 — OBD2 code P0426 flags a problem with the catalytic converter efficiency monitoring (catalyst temperature sensor range/performance (bank 1, sensor 1)). The car is usually drivable while you plan the repair. Common culprits: faulty downstream or upstream oxygen sensor, aged or contaminated catalytic converter, exhaust leak ahead of the converter.
• P0427 — P0427 — Catalyst Temperature Sensor Low (Bank 1, Sensor 1). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: oil or coolant entering the exhaust, exhaust leak ahead of the converter, aged or contaminated catalytic converter.
• P0428 — Catalyst Temperature Sensor High (Bank 1, Sensor 1) (P0428): a powertrain (engine/transmission) fault affecting the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Most often caused by aged or contaminated catalytic converter, engine misfires damaging the catalyst, exhaust leak ahead of the converter.
• P0429 — P0429 — Catalyst Heater Control Circuit (Bank 1). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: faulty downstream or upstream oxygen sensor, engine misfires damaging the catalyst, persistent rich or lean fuel mixture.
• P042A — OBD2 code P042A flags a problem with the catalytic converter efficiency monitoring (catalyst temperature sensor circuit (bank 1 sensor 2)). The car is usually drivable while you plan the repair. Common culprits: faulty downstream or upstream oxygen sensor, engine misfires damaging the catalyst, exhaust leak ahead of the converter.
• P042B — Catalyst Temperature Sensor Circuit Range/Performance (Bank 1 Sensor 2) (P042B): a powertrain (engine/transmission) fault affecting the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Most often caused by oil or coolant entering the exhaust, aged or contaminated catalytic converter, persistent rich or lean fuel mixture.
• P042C — OBD2 code P042C flags a problem with the catalytic converter efficiency monitoring (catalyst temperature sensor circuit low (bank 1 sensor 2)). The car is usually drivable while you plan the repair. Common culprits: oil or coolant entering the exhaust, engine misfires damaging the catalyst, persistent rich or lean fuel mixture.
• P042D — OBD2 code P042D flags a problem with the catalytic converter efficiency monitoring (catalyst temperature sensor circuit high (bank 1 sensor 2)). The car is usually drivable while you plan the repair. Common culprits: engine misfires damaging the catalyst, exhaust leak ahead of the converter, oil or coolant entering the exhaust.
• P042E — OBD2 code P042E flags a problem with the exhaust gas recirculation (EGR) system (exhaust gas recirculation "A" control stuck open). The car is usually drivable while you plan the repair. Common culprits: carbon buildup blocking the EGR valve or passages, vacuum leaks in EGR control lines, ECM control circuit fault.
• P042F — Exhaust Gas Recirculation "A" Control Stuck Closed (P042F): a powertrain (engine/transmission) fault affecting the exhaust gas recirculation (EGR) system. The car is usually drivable while you plan the repair. Most often caused by faulty EGR solenoid or wiring, failed EGR valve or position sensor, carbon buildup blocking the EGR valve or passages.
• P0430 — P0430 — Catalyst System Efficiency Below Threshold (Bank 2). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: persistent rich or lean fuel mixture, exhaust leak ahead of the converter, faulty downstream or upstream oxygen sensor.
• P0431 — OBD2 code P0431 flags a problem with the catalytic converter efficiency monitoring (warm up catalyst efficiency below threshold (bank 2)). The car is usually drivable while you plan the repair. Common culprits: oil or coolant entering the exhaust, persistent rich or lean fuel mixture, faulty downstream or upstream oxygen sensor.
• P0432 — P0432 — Main Catalyst Efficiency Below Threshold (Bank 2). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: persistent rich or lean fuel mixture, oil or coolant entering the exhaust, faulty downstream or upstream oxygen sensor.
• P0433 — Heated Catalyst Efficiency Below Threshold (Bank 2) (P0433): a powertrain (engine/transmission) fault affecting the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Most often caused by faulty downstream or upstream oxygen sensor, oil or coolant entering the exhaust, engine misfires damaging the catalyst.
• P0434 — Heated Catalyst Temperature Below Threshold (Bank 2) (P0434): a powertrain (engine/transmission) fault affecting the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Most often caused by exhaust leak ahead of the converter, engine misfires damaging the catalyst, faulty downstream or upstream oxygen sensor.
• P0435 — Catalyst Temperature Sensor Circuit Malfunction (Bank 2, Sensor 1) (P0435): a powertrain (engine/transmission) fault affecting the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Most often caused by aged or contaminated catalytic converter, engine misfires damaging the catalyst, faulty downstream or upstream oxygen sensor.
• P0436 — P0436 — Catalyst Temperature Sensor Circuit Range/Performance (Bank 2, Sensor 1). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: engine misfires damaging the catalyst, exhaust leak ahead of the converter, aged or contaminated catalytic converter.
• P0437 — P0437 — Catalyst Temperature Sensor Circuit Low (Bank 2, Sensor 1). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: engine misfires damaging the catalyst, exhaust leak ahead of the converter, aged or contaminated catalytic converter.
• P0438 — OBD2 code P0438 flags a problem with the catalytic converter efficiency monitoring (catalyst temperature sensor circuit high (bank 2, sensor 1)). The car is usually drivable while you plan the repair. Common culprits: faulty downstream or upstream oxygen sensor, exhaust leak ahead of the converter, aged or contaminated catalytic converter.
• P0439 — OBD2 code P0439 flags a problem with the catalytic converter efficiency monitoring (catalyst heater control circuit (bank 2)). The car is usually drivable while you plan the repair. Common culprits: persistent rich or lean fuel mixture, oil or coolant entering the exhaust, faulty downstream or upstream oxygen sensor.
• P043A — P043A — Catalyst Temperature Sensor Circuit (Bank 2 Sensor 2). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: aged or contaminated catalytic converter, exhaust leak ahead of the converter, persistent rich or lean fuel mixture.
• P043B — P043B — Catalyst Temperature Sensor Circuit Range/Performance Bank 2 Sensor 2). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: exhaust leak ahead of the converter, aged or contaminated catalytic converter, persistent rich or lean fuel mixture.
• P043C — P043C — Catalyst Temperature Sensor Circuit Low (Bank 2 Sensor 2). A fault in the catalytic converter efficiency monitoring. The car is usually drivable while you plan the repair. Typical causes: persistent rich or lean fuel mixture, faulty downstream or upstream oxygen sensor, oil or coolant entering the exhaust.