OBD2 Fault Code Dictionary: Common P-Codes Explained (UK)

Handheld OBD2 scanner plugged into a UK car's diagnostic port reading fault codes

When the engine warning light comes on, an OBD2 scanner reads a five-character fault code that tells you which system is unhappy. This dictionary explains the most common generic P0XXX powertrain codes in plain English, with the likely causes and the parts worth checking. Codes here are generic (defined by the standard) and apply across makes, though always confirm against your vehicle before buying.

How to read an OBD2 code

Every code has the format P0171: the letter is the system (P = powertrain, B = body, C = chassis, U = network); the first digit shows whether it is generic (0) or manufacturer-specific (1); and the remaining digits identify the fault. A code points you to a circuit or symptom, not always the failed part — diagnosis still matters. To pull codes you need a scanner; see our OBD2 scanners range for handheld and Bluetooth options.

Common generic P-codes reference table

Code Meaning Likely causes Parts to check
P0010 Camshaft position actuator circuit (Bank 1) VVT solenoid fault, low oil pressure, wiring Camshaft actuator/VVT solenoid, wiring
P0011 Camshaft position timing over-advanced (Bank 1) Stuck VVT solenoid, timing error VVT solenoid, timing chain/tensioner
P0016 Crankshaft/camshaft position correlation (Bank 1) Stretched timing chain, sensor fault Timing chain, cam/crank sensors
P0030 HO2S heater control circuit (Bank 1 Sensor 1) Failed oxygen sensor heater, wiring, fuse Upstream lambda sensor, fuse
P0101 Mass air flow circuit range/performance Dirty or faulty MAF, air leak MAF sensor, air filter, intake hoses
P0102 MAF circuit low input Sensor fault, wiring, blocked filter MAF sensor, wiring
P0113 Intake air temperature sensor high input Faulty IAT sensor, open circuit IAT sensor, wiring
P0118 Engine coolant temperature sensor high input Faulty ECT sensor, wiring Coolant temperature sensor
P0128 Coolant temperature below thermostat regulating temp Thermostat stuck open, faulty ECT sensor Thermostat, coolant temperature sensor
P0131 O2 sensor circuit low voltage (Bank 1 Sensor 1) Lean condition, failed sensor, wiring Upstream lambda sensor
P0133 O2 sensor slow response (Bank 1 Sensor 1) Ageing/contaminated sensor Upstream lambda sensor
P0134 O2 sensor no activity (Bank 1 Sensor 1) Dead sensor, broken wiring Upstream lambda sensor, wiring
P0171 System too lean (Bank 1) Vacuum/air leak, weak fuel pump, dirty MAF Intake hoses, MAF, fuel pump, lambda sensor
P0172 System too rich (Bank 1) Leaking injector, faulty MAF, high fuel pressure Injectors, MAF, fuel pressure regulator
P0174 System too lean (Bank 2) Air leak, weak fuel delivery Intake hoses, MAF, fuel pump
P0300 Random/multiple cylinder misfire Worn plugs/coils, vacuum leak, fuel issue Spark plugs, ignition coils, injectors
P0301 Cylinder 1 misfire Plug, coil or injector on cyl 1 Spark plug, coil, injector (cyl 1)
P0302 Cylinder 2 misfire Plug, coil or injector on cyl 2 Spark plug, coil, injector (cyl 2)
P0303 Cylinder 3 misfire Plug, coil or injector on cyl 3 Spark plug, coil, injector (cyl 3)
P0304 Cylinder 4 misfire Plug, coil or injector on cyl 4 Spark plug, coil, injector (cyl 4)
P0305 Cylinder 5 misfire Plug, coil or injector on cyl 5 Spark plug, coil, injector (cyl 5)
P0306 Cylinder 6 misfire Plug, coil or injector on cyl 6 Spark plug, coil, injector (cyl 6)
P0307 Cylinder 7 misfire Plug, coil or injector on cyl 7 Spark plug, coil, injector (cyl 7)
P0308 Cylinder 8 misfire Plug, coil or injector on cyl 8 Spark plug, coil, injector (cyl 8)
P0325 Knock sensor circuit (Bank 1) Faulty knock sensor, wiring Knock sensor, wiring
P0335 Crankshaft position sensor circuit Failed sensor, damaged reluctor ring, wiring Crankshaft position sensor
P0340 Camshaft position sensor circuit Failed sensor, wiring, timing fault Camshaft position sensor
P0401 EGR flow insufficient Blocked EGR valve/passages, faulty valve EGR valve, EGR passages
P0420 Catalyst efficiency below threshold (Bank 1) Worn catalytic converter, faulty O2 sensor, exhaust leak Catalytic converter, downstream lambda sensor
P0430 Catalyst efficiency below threshold (Bank 2) Worn cat, faulty O2 sensor Catalytic converter (Bank 2), lambda sensor
P0440 EVAP system fault (general) Loose/faulty fuel cap, EVAP leak Fuel filler cap, EVAP hoses, purge valve
P0442 EVAP system small leak detected Small leak, loose fuel cap Fuel cap, EVAP lines
P0455 EVAP system large leak detected Missing/loose fuel cap, split EVAP hose Fuel filler cap, EVAP hoses, purge/vent valve
P0500 Vehicle speed sensor malfunction Failed VSS, wiring, ABS fault Vehicle speed sensor, wiring

How to use this dictionary

Pull the code, look it up above, then work from the most likely cause first. For misfires (P0300-P0308) the cheapest checks come first: inspect and replace worn spark plugs and ignition coils before suspecting injectors. A P0171 lean code is most often a vacuum or intake leak rather than a failed sensor, so check hoses before spending on parts. Catalyst codes like P0420 frequently start with a tired downstream lambda sensor or an exhaust leak, not always a new cat.

A note on clearing codes

Clearing a code does not fix the fault — if the cause remains, it will return after a few drive cycles. Always diagnose and repair, then clear and road-test to confirm the light stays off. Some codes (especially EVAP and catalyst monitors) need a full drive cycle to re-run their self-test before they confirm a fix.

Codes are not always engine-related

A warning light can also flag chassis and braking faults. If your scanner shows ABS or brake-related C-codes, or you notice poor stopping performance, inspect and renew your brake pads and discs as part of the same service. A quality OBD2 reader from our scanner range lets you check live data and monitor readiness before an MOT. Use code SAVE10 for 10% off, with tracked UK delivery.

Frequently asked questions

What does a P-code starting with P0 mean?
The P means it is a powertrain code and the 0 means it is a generic code defined by the OBD2 standard, so it applies across makes. A first digit of 1 (e.g. P1xxx) means it is manufacturer-specific.
Is it safe to drive with a P0420 catalytic converter code?
P0420 usually will not leave you stranded, but it indicates the catalytic converter is no longer cleaning the exhaust efficiently. It will fail an MOT emissions test and can worsen over time, so it should be diagnosed promptly. Check the downstream lambda sensor and exhaust for leaks first.
Why does my fault code come back after I clear it?
Clearing a code only resets the warning light; it does not repair the fault. If the underlying problem remains, the code returns after a few drive cycles. Diagnose and fix the cause, then clear the code and road-test to confirm it stays off.