Car Fault Codes Explained
A scanner or a garage has given you a code like P0420 and you want to know what it means. Find it below for a plain-English guide: what has gone wrong, whether you can keep driving, what you can check yourself and roughly what the fix costs in Ireland and the UK.
How to read a fault code
PowertrainEngine, fuel, exhaust and gearbox. Most codes you will see start with P.
BodyAirbags, seatbelts and the body electrics.
ChassisBrakes, ABS, steering and suspension.
NetworkThe car's computers losing contact with each other.
A 0 or 2 after the letter means a standard code that means the same on every car; a 1 means the maker's own code, so its meaning depends on the make. Some scanners add two digits for the type of failure - P200200 is P2002.
Engine air, fuel and sensors12
P01, P02, P05 and P2 codes: the air and fuel the engine gets, turbo boost, idle and engine sensors.
- P0101
Mass Air Flow (MAF) Circuit Range/Performance
The engine computer is receiving unexpected airflow readings from the mass air flow sensor.
Service soon
- P0113
Intake Air Temperature Sensor 1 Circuit High Input
The car computer is reading an unusually high voltage signal from the intake air temperature sensor, which usually means an open circuit or unplugged wire.
Service soon
- P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
This code means your engine is not warming up to its proper operating temperature within the expected time.
Service soon
- P0135
O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
The internal electric heater in your engine's front oxygen sensor has stopped working properly.
Service soon
- P0141
O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
The heating element inside the rear oxygen sensor has stopped working properly.
Service soon
- P0171
System Too Lean (Bank 1)
This code means the engine is getting too much air or not enough petrol on bank one.
Service soon
- P0174
System Too Lean (Bank 2)
This code means the cylinders on bank two are receiving too much air or not enough petrol.
Service soon
- P0299
Turbocharger / Supercharger 'A' Underboost Condition
Your engine computer detected lower pressure from the turbocharger than it asked for.
Service soon
- P0507
Idle Control System RPM Higher Than Expected
Your engine is revving faster than normal when parked or idling.
Service soon
- P0520
Engine Oil Pressure Sensor / Switch Circuit Malfunction
Fault code P0520 means the engine computer has detected an electrical fault in the oil pressure sensor circuit.
Stop driving
- P2015
Intake Manifold Runner Position Sensor / Switch Circuit Range/Performance
A sensor detected that the airflow flaps inside your engine's intake manifold are turning past their normal limits.
Service soon
- P2178
System Too Rich Off Idle (Bank 1)
Your engine computer detects too much petrol compared to air while you are driving above idle.
Service soon
Misfire, ignition and timing7
P03 codes: spark, misfires and the sensors that time the engine. A flashing engine light here means stop.
- P0300
Random / Multiple Cylinder Misfire Detected
Fault code P0300 means the engine computer has detected incomplete combustion across more than one cylinder.
Service soon
- P0325
Knock Sensor 1 Circuit Malfunction (Bank 1)
The engine computer has lost reliable communication with knock sensor 1.
Service soon
- P0335
Crankshaft Position Sensor 'A' Circuit Malfunction
Your engine computer has lost the signal that tells it how fast the crankshaft is turning and where the pistons are.
Stop driving
- P0340
Camshaft Position Sensor 'A' Circuit Malfunction (Bank 1)
The engine computer has lost the electrical signal from the camshaft position sensor on bank 1.
Service soon
- P0351
Ignition Coil 'A' Primary/Secondary Circuit Malfunction
The engine computer has detected an electrical circuit failure at the ignition coil for cylinder one.
Service soon
- P0392
Camshaft Position Sensor 'B' Circuit Low (Bank 2)
The engine computer detects that the exhaust camshaft sensor on the second cylinder bank is sending an abnormally low voltage signal.
Service soon
- P0393
Camshaft Position Sensor 'B' Circuit High (Bank 2)
The engine computer is detecting an unusually high voltage signal from the exhaust camshaft sensor on cylinder bank two.
Service soon
Exhaust and emissions8
P04 and DPF codes: catalytic converter, EGR, fuel vapour and the diesel particulate filter.
- P0401
Exhaust Gas Recirculation (EGR) Flow Insufficient Detected
The exhaust gas recirculation system is not passing enough burnt gas back into the engine to cool combustion.
Service soon
- P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Your engine computer has detected that the catalytic converter is not cleaning exhaust gases as efficiently as it should.
Service soon
- P0430
Catalyst System Efficiency Below Threshold (Bank 2)
Your engine computer has detected that the catalytic converter on bank two is not cleaning exhaust gases effectively.
Service soon
- P0442
Evaporative Emission Control System Leak Detected (Small Leak)
The engine computer has detected a tiny escape of petrol fumes from the fuel vapour recovery system.
Service soon
- P0446
EVAP Vent Control Circuit Malfunction
A valve controlling the release of fuel vapour from your charcoal filter has an electrical or flow fault.
Service soon
- P0455
EVAP System Emission Control System Leak Detected (Gross Leak)
Your car has detected a substantial fuel vapour leak in the evaporative emissions system.
Service soon
- P0456
EVAP System Very Small Leak Detected
Your car has detected a tiny escape of petrol fumes from the emissions vapour system, often around the fuel cap or a hose.
Service soon
- P2002
Diesel Particulate Filter Efficiency Below Threshold (Bank 1)
The engine computer has detected that the diesel particulate filter is not catching soot as well as it should.
Service soon
Control modules and gearbox5
U codes (network) and gearbox codes: the car's computers losing contact or reporting a fault.
- P0700
Transmission Control System Malfunction
This code means the gearbox computer has detected a fault and asked the main engine computer to turn on the warning light.
Service soon
- U0100
Lost Communication With ECM / PCM 'A'
Your car's other computers have lost contact with the main engine control module.
Stop driving
- U0101
Lost Communication With Transmission Control Module (TCM)
The main engine computer has lost contact with the computer that controls your gearbox.
Service soon
- U0121
Lost Communication With Anti-Lock Brake System (ABS) Control Module
Your car's main computer has lost contact with the anti-lock brake system module.
Service soon
- U0155
Lost Communication With Instrument Panel Cluster (IPC) Control Module
Your car computers have lost contact with the instrument cluster on the dashboard.
Service soon
Brakes and ABS4
C codes (chassis): wheel speed sensors and the ABS unit. Normal braking usually stays, the anti-lock help may not.
- C0035
Left Front Wheel Speed Sensor Circuit Malfunction
This code means the anti-lock braking computer has lost a reliable signal from the sensor on the front left wheel.
Service soon
- C0040
Right Front Wheel Speed Sensor Circuit Malfunction
Your car cannot read the speed of the front right wheel due to an electrical circuit fault.
Service soon
- C0245
Wheel Speed Sensor Frequency Error
The ABS computer is receiving an erratic or inconsistent speed reading from one of your wheels.
Service soon
- C0550
ABS Control Module (ECU) Internal Fault
The computer that controls your anti-lock braking system has detected an internal electronic failure.
Service soon
Airbags and body electrics4
B codes (body): airbags, the body control module and the car's identity.
- B0010
Driver Frontal Stage 1 Deployment Control Circuit Malfunction
Fault code B0010 means the car computer has detected an electrical fault in the driver front airbag circuit.
Service soon
- B0100
Side Airbag Deployment Control Circuit Malfunction
Fault code B0100 means the car detected an electrical problem in a side airbag circuit.
Service soon
- B1318
Battery Voltage Low (Body Control Module)
The body control computer has detected that the electrical system voltage dropped below a safe level.
Service soon
- B2900
VIN Mismatch / Invalid Vehicle Identification Number
A computer module inside the car does not recognise the Vehicle Identification Number programmed into it.
Stop driving
Hybrid and electric12
High-voltage battery, motor and inverter faults. Checks only - the repairs are for trained technicians.
- P0A09
DC/DC Converter Status Circuit Low (12V Charging, Hybrid/EV)
Your hybrid or electric car's DC/DC converter has stopped charging the standard 12-volt battery.
Stop drivingHybrid & EV
- P0A0F
Engine Failed to Start (Hybrid)
This hybrid fault code means the petrol engine turned over but failed to fire and run.
Stop drivingHybrid & EV
- P0A3F
Drive Motor 'A' Position Sensor Circuit (Hybrid/EV)
This code means the car computer cannot track the exact position of the electric drive motor.
Stop drivingHybrid & EV
- P0A78
Drive Motor 'A' Inverter Performance (Hybrid/EV)
The car's computer has detected an electrical fault inside the inverter that powers the main electric motor.
Stop drivingHybrid & EV
- P0A7E
Hybrid Battery Pack Over Temperature
Your car's computer has detected that the high-voltage hybrid battery pack is running too hot.
Stop drivingHybrid & EV
- P0A7F
Hybrid Battery Pack Deterioration
The car computer has detected that the high-voltage hybrid battery pack has degraded and lost its capacity to hold an even charge.
Service soonHybrid & EV
- P0A80
Replace Hybrid Battery Pack
This code means the hybrid computer has detected a failing cell or voltage imbalance inside the high-voltage battery pack.
Service soonHybrid & EV
- P0A93
Inverter Cooling System Performance (Hybrid)
Your hybrid system has detected that its separate inverter cooling system is not working properly.
Stop drivingHybrid & EV
- P0A94
DC/DC (Boost) Converter Performance (Hybrid)
This code means the hybrid boost converter inside the inverter is malfunctioning or overheating.
Stop drivingHybrid & EV
- P0AA6
Hybrid/EV Battery Voltage System Isolation Fault
A safety sensor has detected electrical current leaking from the high-voltage system into the car's metal chassis.
Stop drivingHybrid & EV
- P0AFA
Hybrid/EV Battery System Voltage Low
The car computer detected that the main high-voltage battery pack dropped below safe operating voltage.
Stop drivingHybrid & EV
- U0293
Lost Communication With Hybrid/EV Powertrain Control Module
The car computer has lost contact with the hybrid or electric powertrain controller.
Stop drivingHybrid & EV
Code not listed, or more than one?
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