Fault codes explained
Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.
Find a code to read its plain-English meaning, symptoms, possible causes, safe first checks and broad 2026 UK independent-garage cost range.
Fuel and air
Your engine burns a careful mix of fuel and air. These codes mean a sensor has seen too much or too little of one of them, or a reading that makes no sense. Many are cheap to fix, but they often hurt fuel economy until you do.
- P0100: Mass or volume airflow “A” circuit malfunction
The engine control unit (ECU) has detected a general fault in the primary mass/volume airflow sensor circuit.
- P0101: Mass or volume airflow “A” circuit range/performance
The airflow signal is present, but the ECU considers it implausible for the engine’s speed, load, throttle position or other operating conditions.
- P0102: Mass or volume airflow “A” circuit low input
The ECU is seeing an airflow-sensor signal below its expected electrical or frequency threshold.
- P0103: Mass or volume airflow “A” circuit high input
The ECU is seeing an airflow-sensor signal above the permitted electrical or frequency threshold.
- P0104: Mass or volume airflow “A” circuit intermittent/erratic
The airflow signal is dropping out, spiking or becoming implausibly unstable instead of staying consistently low or high.
- P0110: Intake-air temperature sensor 1 circuit
The ECU has detected a general electrical fault in the intake-air-temperature (IAT) sensor 1 circuit.
- P0111: Intake-air temperature sensor 1 circuit range/performance
The IAT circuit is producing a signal, but the ECU considers the reading implausible, too slow to change or inconsistent with expected operating conditions.
- P0112: Intake-air temperature sensor 1 circuit low input
The ECU is seeing abnormally low signal voltage from the IAT circuit.
- P0113: Intake-air temperature sensor 1 circuit high input
The ECU is seeing abnormally high signal voltage from the IAT circuit.
- P0115: Engine coolant temperature sensor 1 circuit
The ECU has detected a general electrical fault in the engine-coolant-temperature (ECT) sensor 1 circuit.
- P0116: Engine coolant temperature circuit range/performance
The engine computer can see a coolant-temperature signal, but the reading is not changing in the expected way as the engine warms or cools.
- P0117: Engine coolant temperature circuit low input
The engine computer is seeing voltage below the permitted range in the coolant-temperature circuit.
- P0118: Engine coolant temperature circuit high input
Voltage in the coolant-temperature circuit is above the range the engine computer considers credible.
- P0120: Throttle/pedal position sensor or switch “A” circuit
The engine computer has detected an electrical fault in throttle or accelerator-position circuit “A”.
- P0121: Throttle/pedal position sensor or switch “A” range/performance
The position signal is present, but the engine computer considers it stuck, outside the expected operating relationship or inconsistent with another throttle signal or engine data.
- P0122: Throttle/pedal position sensor or switch “A” circuit low
The engine computer is receiving voltage below the accepted range from throttle or accelerator-position circuit “A”.
- P0123: Throttle/pedal position sensor or switch “A” circuit high
The control unit is seeing voltage above the permitted range from throttle or accelerator-position circuit “A”.
- P0128: Coolant temperature below thermostat regulating temperature
After allowing for the starting temperature and how the car has been driven, the engine computer expected the coolant to become warmer than the measured value.
- P0130: Oxygen/lambda sensor circuit malfunction, Bank 1 Sensor 1
The engine computer is not receiving a plausible or dependable signal from the upstream oxygen, or lambda, sensor circuit on Bank 1.
- P0131: Oxygen/lambda sensor circuit low voltage, Bank 1 Sensor 1
The upstream lambda-sensor signal on Bank 1 has stayed below the manufacturer’s accepted voltage.
- P0132: Oxygen sensor circuit high voltage, Bank 1 Sensor 1
The engine computer is seeing an unusually high signal from the upstream oxygen or air/fuel sensor circuit on Bank 1.
- P0133: Oxygen sensor circuit slow response, Bank 1 Sensor 1
The upstream Bank 1 sensor is responding to mixture changes more slowly than the computer expects.
- P0134: Oxygen sensor circuit no activity detected, Bank 1 Sensor 1
The computer cannot see useful activity from the upstream Bank 1 sensor when it expects the sensor to be working.
- P0135: Oxygen sensor heater circuit malfunction, Bank 1 Sensor 1
The computer has detected an electrical or warm-up problem in the heater circuit for the upstream Bank 1 sensor.
- P0136: Oxygen sensor circuit malfunction, Bank 1 Sensor 2
The computer has detected a general signal or circuit fault for the downstream Bank 1 sensor, normally after the catalytic converter.
- P0137: Oxygen sensor circuit low voltage, Bank 1 Sensor 2
The downstream Bank 1 sensor signal has remained below the range the computer expects.
- P0138: Oxygen sensor circuit high voltage, Bank 1 Sensor 2
The downstream Bank 1 sensor signal is staying above the computer's calibrated high limit.
- P0140: Oxygen sensor circuit no activity detected, Bank 1 Sensor 2
The computer has not seen the expected response from the downstream Bank 1 sensor during its monitor.
- P0141: Oxygen sensor heater circuit malfunction, Bank 1 Sensor 2
The computer has detected an electrical problem with the heater for the downstream Bank 1 sensor.
- P0171: Fuel-trim system too lean, Bank 1
The computer has had to add more fuel than its normal correction range to counter a lean indication on Bank 1.
- P0172: System too rich (Bank 1)
The engine control unit (ECU) has reached the limit of how much fuel it can subtract, yet Bank 1 — the side containing cylinder 1 — still appears to have too much fuel for the available air.
- P0174: System too lean (Bank 2)
The ECU has reached the limit of how much fuel it can add, but Bank 2 — normally the cylinder bank opposite the one containing cylinder 1 — still appears to have too much air or too little fuel.
- P0175: System too rich (Bank 2)
The ECU is subtracting fuel but Bank 2 still appears too rich.
- P0190: Fuel rail pressure sensor “A” circuit
The ECU cannot trust the electrical circuit used by fuel-rail pressure sensor “A”.
- P0191: Fuel rail pressure sensor “A” range/performance
The ECU receives a rail-pressure signal, but the reading is implausible or does not respond as expected.
- P0192: Fuel rail pressure sensor “A” circuit low
The ECU sees the fuel-rail pressure sensor signal below its permitted electrical range.
- P0193: Fuel rail pressure sensor “A” circuit high
The ECU sees the fuel-rail pressure sensor signal above its permitted electrical range.
- P0200: Injector circuit malfunction
The ECU has detected an electrical fault in the fuel-injector control system, but this generic code does not identify one cylinder.
- P0201: Cylinder 1 injector circuit malfunction
The ECU has detected an abnormal electrical condition in the injector circuit assigned to cylinder 1.
- P0202: Cylinder 2 injector circuit malfunction
The ECU has detected an electrical fault in the injector circuit for cylinder 2.
- P0203: Injector circuit fault, cylinder 3
The engine control unit has detected an electrical problem in the control circuit for cylinder 3's fuel injector.
- P0204: Injector circuit fault, cylinder 4
The engine control unit has found an electrical problem in the circuit operating cylinder 4's fuel injector.
Engine and cooling
Misfires, timing problems and temperature faults. Some of these can wreck an engine if you keep driving, so each page says plainly whether it is safe to carry on.
- P0217: Engine over-temperature condition
The control unit has recorded engine coolant temperature above its permitted threshold.
- P0300: Random or multiple-cylinder misfire detected
The engine control unit has detected misfiring that affects more than one cylinder, occurs irregularly, or cannot yet be assigned reliably to one cylinder.
- P0301: Cylinder 1 misfire detected
The engine control unit has detected that cylinder 1 is not contributing consistently.
- P0302: Cylinder 2 misfire detected
The engine control unit has recognised repeated or significant misfire activity associated with cylinder 2.
- P0303: Cylinder 3 misfire detected
P0303 means the engine control unit has detected a misfire associated with cylinder 3.
- P0304: Cylinder 4 misfire detected
The engine control unit has detected that cylinder 4 failed to contribute normally during some combustion events.
- P0305: Cylinder 5 misfire detected
The engine computer has detected that cylinder 5 is not contributing evenly.
- P0306: Cylinder 6 misfire detected
The engine computer has detected an uneven contribution from cylinder 6.
- P0325: Knock sensor 1 circuit malfunction
The control module has found an electrical fault or implausible signal in knock-sensor circuit 1, usually for Bank 1 or the sole sensor.
- P0335: Crankshaft position sensor A circuit malfunction
The engine computer is not receiving the expected signal from crankshaft-position sensor circuit A.
- P0340: Camshaft position sensor A circuit malfunction
The control module has detected a missing or invalid signal in camshaft-position sensor A’s circuit, generally for Bank 1 or a single-sensor layout.
Exhaust, turbo and emissions
Catalytic converter, EGR and turbo codes. They can mean an MOT emissions fail, and a turbo fault can put the car into limp mode. Most start with a cheap check before anything expensive.
- P0234: Turbocharger or supercharger overboost condition
The engine control unit has measured, or believes it has measured, more boost pressure than it commanded.
- P0299: Turbocharger or supercharger underboost condition
The engine control unit has detected less boost pressure than it requested under the relevant operating conditions.
- P0400: Exhaust gas recirculation flow malfunction
The engine computer has detected that exhaust gas recirculation (EGR) flow is not behaving as expected.
- P0401: Exhaust gas recirculation flow insufficient detected
The control module commanded EGR operation but measured or inferred less flow than expected.
- P0402: Exhaust gas recirculation flow excessive detected
The engine computer has measured or inferred more EGR flow than it commanded.
- P0420: Catalyst system efficiency below threshold, Bank 1
The control module believes the Bank 1 catalytic converter is storing and treating oxygen less effectively than its calibrated threshold.
- P0430: Catalyst system efficiency below threshold, Bank 2
The control module believes the catalytic converter on Bank 2 is working below its calibrated efficiency threshold.