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

• P040D — OBD2 code P040D flags a problem with the exhaust gas recirculation (EGR) system (exhaust gas recirculation temperature sensor "A" circuit high). The car is usually drivable while you plan the repair. Common culprits: carbon buildup blocking the EGR valve or passages, faulty EGR solenoid or wiring, failed EGR valve or position sensor.
• P040E — P040E — Exhaust Gas Recirculation Temperature Sensor "A" Circuit Intermittent/Erratic. A fault in the exhaust gas recirculation (EGR) system. The car is usually drivable while you plan the repair. Typical causes: carbon buildup blocking the EGR valve or passages, failed EGR valve or position sensor, vacuum leaks in EGR control lines.
• P040F — OBD2 code P040F flags a problem with the exhaust gas recirculation (EGR) system (exhaust gas recirculation temperature sensor "A"/"B" correlation). The car is usually drivable while you plan the repair. Common culprits: faulty EGR solenoid or wiring, carbon buildup blocking the EGR valve or passages, vacuum leaks in EGR control lines.
• P0410 — OBD2 code P0410 flags a problem with the sensor and actuator circuits (secondary air injection system malfunction). The car is usually drivable while you plan the repair. Common culprits: blown fuse feeding the circuit, corroded, loose or damaged connector, open or shorted wiring in the circuit.
• P0411 — OBD2 code P0411 flags a problem with the sensor and actuator circuits (secondary air injection system incorrect flow detected). The car is usually drivable while you plan the repair. Common culprits: control module driver fault, poor ground connection, blown fuse feeding the circuit.
• P0412 — P0412 — Secondary Air Injection System Switching Valve A Circuit Malfunction. A fault in the sensor and actuator circuits. The car is usually drivable while you plan the repair. Typical causes: poor ground connection, corroded, loose or damaged connector, blown fuse feeding the circuit.
• P0413 — OBD2 code P0413 flags a problem with the sensor and actuator circuits (secondary air injection system switching valve a circuit open). The car is usually drivable while you plan the repair. Common culprits: open or shorted wiring in the circuit, faulty sensor or actuator of the affected circuit, corroded, loose or damaged connector.
• P0414 — P0414 — Secondary Air Injection System Switching Valve A Circuit Shorted. A fault in the sensor and actuator circuits. The car is usually drivable while you plan the repair. Typical causes: corroded, loose or damaged connector, faulty sensor or actuator of the affected circuit, blown fuse feeding the circuit.
• P0415 — OBD2 code P0415 flags a problem with the sensor and actuator circuits (secondary air injection system switching valve b circuit malfunction). The car is usually drivable while you plan the repair. Common culprits: poor ground connection, control module driver fault, corroded, loose or damaged connector.
• P0416 — Secondary Air Injection System Switching Valve B Circuit Open (P0416): a powertrain (engine/transmission) fault affecting the sensor and actuator circuits. The car is usually drivable while you plan the repair. Most often caused by open or shorted wiring in the circuit, faulty sensor or actuator of the affected circuit, poor ground connection.
• P0417 — P0417 — Secondary Air Injection System Switching Valve B Circuit Shorted. A fault in the sensor and actuator circuits. The car is usually drivable while you plan the repair. Typical causes: open or shorted wiring in the circuit, blown fuse feeding the circuit, corroded, loose or damaged connector.
• P0418 — Secondary Air Injection System Relay "A" Circuit Malfunction (P0418): a powertrain (engine/transmission) fault affecting the sensor and actuator circuits. The car is usually drivable while you plan the repair. Most often caused by open or shorted wiring in the circuit, corroded, loose or damaged connector, control module driver fault.
• P0419 — P0419 — Secondary Air Injection System Relay "B" Circuit Malfunction. A fault in the sensor and actuator circuits. The car is usually drivable while you plan the repair. Typical causes: faulty sensor or actuator of the affected circuit, control module driver fault, open or shorted wiring in the circuit.
• P041A — P041A — Exhaust Gas Recirculation Temperature Sensor "B" Circuit. A fault in the exhaust gas recirculation (EGR) system. The car is usually drivable while you plan the repair. Typical causes: vacuum leaks in EGR control lines, ECM control circuit fault, carbon buildup blocking the EGR valve or passages.
• P041B — P041B — Exhaust Gas Recirculation Temperature Sensor "B" Circuit Range/Performance. A fault in the exhaust gas recirculation (EGR) system. The car is usually drivable while you plan the repair. Typical causes: carbon buildup blocking the EGR valve or passages, ECM control circuit fault, faulty EGR solenoid or wiring.
• P041C — OBD2 code P041C flags a problem with the exhaust gas recirculation (EGR) system (exhaust gas recirculation temperature sensor "B" circuit low). The car is usually drivable while you plan the repair. Common culprits: ECM control circuit fault, failed EGR valve or position sensor, vacuum leaks in EGR control lines.
• P041D — Exhaust Gas Recirculation Temperature Sensor "B" Circuit High (P041D): 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 ECM control circuit fault, vacuum leaks in EGR control lines, failed EGR valve or position sensor.
• P041E — Exhaust Gas Recirculation Temperature Sensor "B" Circuit Intermittent/Erratic (P041E): 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 vacuum leaks in EGR control lines, ECM control circuit fault, faulty EGR solenoid or wiring.
• P041F — P041F — Secondary Air Injection System Switching Valve "A" Circuit Low. A fault in the sensor and actuator circuits. The car is usually drivable while you plan the repair. Typical causes: faulty sensor or actuator of the affected circuit, open or shorted wiring in the circuit, control module driver fault.
• P0420 — Catalyst System Efficiency Below Threshold (Bank 1) (P0420): 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, faulty downstream or upstream oxygen sensor.
• P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1) (P0421): 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, exhaust leak ahead of the converter, engine misfires damaging the catalyst.
• P0422 — Main Catalyst Efficiency Below Threshold (Bank 1) (P0422): 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 engine misfires damaging the catalyst, faulty downstream or upstream oxygen sensor, aged or contaminated catalytic converter.
• P0423 — OBD2 code P0423 flags a problem with the catalytic converter efficiency monitoring (heated catalyst efficiency below threshold (bank 1)). The car is usually drivable while you plan the repair. Common culprits: faulty downstream or upstream oxygen sensor, aged or contaminated catalytic converter, engine misfires damaging the catalyst.
• P0424 — P0424 — Heated Catalyst Temperature Below Threshold (Bank 1). 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, exhaust leak ahead of the converter.