FAULT CODE LIBRARY

Camshaft position timing over-advanced

The engine computer sees the intake camshaft timing is ahead of where it should be. Usually a stuck or dirty camshaft timing oil control valve, or low/sludged engine oil.

Crankshaft/camshaft position correlation

The crankshaft and camshaft sensors disagree about engine position. Often a stretched timing chain or jumped timing belt — stop driving until inspected.

Mass air flow sensor range/performance

The MAF sensor is reading airflow outside expected range. Usually a dirty or failing MAF sensor, or an intake air leak after the sensor.

Intake air temperature sensor high input

The intake air temperature sensor is sending a signal that reads impossibly cold, typically a disconnected or shorted sensor wiring.

Throttle/pedal position sensor performance

The throttle position readings don't match what the computer expects. Often a worn throttle position sensor or carbon build-up on the throttle body.

Coolant thermostat below regulating temperature

The engine takes too long to reach operating temperature, which usually means the thermostat is stuck open. Fuel economy and heater performance suffer.

Oxygen sensor circuit low voltage (bank 1 sensor 1)

The front oxygen sensor is reporting very low voltage, meaning it may be failing or sensing a genuine lean condition.

Oxygen sensor heater circuit (bank 1 sensor 1)

The heater inside the front oxygen sensor isn't working, so the sensor warms up slowly and fuel trims stay inaccurate until the engine is hot.

Oxygen sensor high voltage (bank 1 sensor 2)

The rear oxygen sensor reports the mixture is constantly rich. Often the sensor itself has failed, or fuel pressure is too high.

Oxygen sensor heater circuit (bank 1 sensor 2)

The heater in the rear oxygen sensor has failed. The car usually drives normally, but emissions tests will fail.

System too lean (bank 1)

The engine is getting too much air or too little fuel on bank 1. Common causes: vacuum leaks, dirty MAF sensor, weak fuel pump or clogged filter.

System too lean (bank 2)

Same lean condition as P0171 but on bank 2 — typical on V6/V8 engines. Look for vacuum leaks, a dirty MAF or low fuel pressure.

Injector circuit open - cylinder 1

The computer can't reach the number 1 injector. Usually a wiring fault, connector issue or failed injector coil.

Engine over-temperature condition

The computer saw coolant temperature in the danger zone. Stop driving — overheating warps heads and blows gaskets. Check coolant level, fan operation and water pump.

Injector circuit low - cylinder 1

The number 1 injector circuit is shorted to ground. Can cause a misfire and raw fuel smell from a partially open injector.

Random/multiple cylinder misfire

The engine is misfiring across several cylinders rather than one. Common causes: worn spark plugs, failing ignition coils, vacuum leaks or low fuel pressure.

Cylinder 1 misfire

Cylinder 1 is not firing properly. Most often a worn spark plug or failing ignition coil, sometimes a clogged injector or low compression.

Cylinder 2 misfire

Cylinder 2 is misfiring. Swap the coil with another cylinder to see if the fault follows the coil — the fastest DIY check.

Cylinder 3 misfire

Cylinder 3 is misfiring. Check the spark plug and coil first, then the injector; persistent misfires can damage the catalytic converter.

Cylinder 4 misfire

Cylinder 4 is misfiring. A flashing check engine light means raw fuel is reaching the catalytic converter — ease off and fix it soon.

Knock sensor circuit low (bank 1)

The knock sensor signal is too weak, so the computer can't detect detonation. Performance and fuel economy drop as timing is kept conservative.

Crankshaft position sensor A circuit

The crankshaft position signal is missing or erratic. The engine may crank but not start, or cut out while driving — treat it as a breakdown risk.

Camshaft position sensor circuit

The camshaft position sensor signal is missing. The engine may run but start poorly and hesitate; most are a simple sensor swap.

Exhaust gas recirculation flow insufficient

The EGR system isn't moving enough exhaust gas back into the engine. Usually carbon clogging in the EGR valve or its passages.

Catalyst efficiency below threshold (bank 1)

The catalytic converter isn't cleaning exhaust efficiently. Causes range from a tired converter to a lazy oxygen sensor, exhaust leak or engine misfires.

Catalyst efficiency below threshold (bank 2)

Same as P0420 but on bank 2. Compare both oxygen sensors; if bank 1 reads healthy, the fault is more likely in the bank 2 converter or its sensor.

EVAP system small leak detected

A small fuel-vapour leak in the EVAP system. Most common culprit: a loose or worn fuel cap — tighten it and see if the code returns.

EVAP system gross leak detected

A large fuel-vapour leak, often a badly seated fuel cap, cracked hose or faulty purge valve. Not immediately dangerous, but fuel vapours escape to atmosphere.

Cooling fan 1 control circuit

The computer can't control cooling fan 1. In traffic the engine can overheat quickly — check the fan fuse, relay, connector and the fan motor itself.

Vehicle speed sensor A

No valid road-speed signal reaching the computer. Speedometer may be dead and automatic shift points become erratic.

Idle air control system

The idle speed control isn't working properly, causing stalling or high/erratic idle. Usually a dirty throttle body or faulty idle air control valve.

Idle air control RPM higher than expected

The engine idles faster than commanded. Often a stuck idle air control valve, vacuum leak or carbon build-up after cleaning the throttle body.

System voltage low

The computer sees charging voltage below normal. Test the battery and alternator; weak charging can cause random electrical faults across the car.

Transmission control system malfunction

A general flag that the transmission computer has stored its own fault codes. The gearbox-specific codes need reading with a scan tool that supports them.

Incorrect gear ratio

The transmission is slipping between gears — the driven ratio doesn't match the commanded one. Check fluid level and condition; continued driving can destroy the gearbox.

Control module power supply interrupted

The engine computer lost power momentarily. Often caused by a weak battery, corroded terminals or a failing ignition switch.

Battery voltage low

A body-electronics module saw battery voltage dip too low. Common on cars with aging batteries or parasitic drains that flatten the battery overnight.

Left front wheel speed sensor circuit

The ABS module lost the left front wheel speed signal. ABS and stability control may deactivate until the sensor or its wiring is repaired.

Right rear wheel speed sensor circuit

A fault in the right rear wheel speed sensor circuit. Expect an ABS warning light; the base brakes still work normally.

Lost communication with engine control module

A module on the network can no longer talk to the engine computer. Causes include wiring damage, corroded connectors or a failing control module — the car may not run reliably.