Car fault codes explained

U0121: lost communication with the ABS module
U0121 means the car's network cannot talk to the ABS module, so anti-lock braking and stability control are switched off. The one code here that is close to a certain MOT failure. Read more...
P0507: idle speed higher than expected
P0507 means idle is higher than target, commonly a vacuum leak or a sticking throttle. It is the one code where the fault itself is a named MOT defect. Free checks first, and indicative UK costs. Read more...
P0506: idle speed lower than expected
P0506 means the engine idles lower than the computer is asking for, often carbon in the throttle body. Why the reset after cleaning matters, and indicative UK costs. Read more...
P0463: fuel level sensor high input
P0463 means the fuel level sender is reading wrong, so the gauge misleads you. More annoyance than danger, with two real consequences worth knowing about. Read more...
P0562: system voltage low
P0562 means charging voltage is low, usually a tired battery or a failing alternator. It is also the code that explains other codes. How to tell which, and indicative UK costs. Read more...
P0341: camshaft position sensor performance
P0341 means the camshaft sensor signal is present but out of range, rather than missing. That difference moves the suspicion from the sensor to what it is reading. How it differs from P0340, and indicative UK costs. Read more...
P0301 to P0304: single-cylinder misfires
P0301, P0302, P0303 and P0304 each name a single misfiring cylinder. Coil, plug or injector? The swap test that answers it on any engine, plus indicative UK costs. Read more...
P0141: oxygen sensor heater circuit (Bank 1 Sensor 2)
P0141 is a heater fault in the rear oxygen sensor. It affects emissions monitoring rather than how the car drives, and fixing it sometimes uncovers a catalyst code hiding behind it. Read more...
P0135: oxygen sensor heater circuit (Bank 1 Sensor 1)
P0135 means the heater inside the front oxygen sensor is not working, so the sensor takes too long to start giving usable readings. Will it fail an MOT, what to check first and indicative UK costs. Read more...
P0133: oxygen sensor slow response (Bank 1 Sensor 1)
P0133 means the front oxygen sensor is still reading correctly but reacting too slowly. Usually an ageing sensor, sometimes a sensor poisoned by something worse. What to check first and indicative UK costs. Read more...
P0172: system too rich (Bank 1)
P0172 means there is too much fuel in the mixture on bank 1. The car may smell of petrol, run rough and drink fuel. The one code most likely to fail the measured emissions test. Read more...
P0174: system too lean (Bank 2)
P0174 is the same lean condition as P0171 but on bank 2. Which banks are flagged is the most useful clue you have. What that tells you, what to check first and indicative UK costs. Read more...
P0113: intake air temperature sensor high input
P0113 means the signal from the intake air temperature sensor is outside the plausible range, often because the sensor is disconnected rather than faulty. Minor, but it affects fuelling. Read more...
P0102: MAF sensor circuit low input
P0102 means the mass airflow sensor is reporting less air than the engine computer believes is plausible. Cleaning often fixes it. The unplug test, what to check first and indicative UK costs. Read more...
P0011: camshaft timing over-advanced (Bank 1)
P0011 means the camshaft on bank 1 is more advanced than the engine computer asked for. Variable valve timing runs on engine oil, so the usual causes are oil related. Causes, what to check first and indicative UK costs. Read more...
P0740: torque converter clutch fault
P0740 means the torque converter lock-up clutch circuit has a fault, usually felt as a judder at a steady cruise in an automatic. The commonest cause is tired fluid rather than a failed converter. Read more...
P0700: transmission control system fault
P0700 does not tell you what is wrong with your gearbox. It is a flag the engine computer stores to say the transmission module has logged a fault of its own. What to do next, and indicative UK costs. Read more...
U0100: lost communication with the ECM or PCM
U0100 means a control module has lost communication with the engine computer over the car's internal network. It often arrives with several warning lights at once. Causes, what to check first and indicative UK costs. Read more...
P0430: catalyst efficiency below threshold (Bank 2)
P0430 is the same test as P0420 but on bank 2, the other cylinder bank of a V6, V8 or flat engine. How it differs, what to check, and indicative UK costs. Read more...
P0401: EGR flow insufficient
P0401 means the exhaust gas recirculation system is not flowing as much as the engine computer expects, nearly always carbon clogging the valve or the passages behind it. Whether cleaning works, the MOT position and indicative UK costs. Read more...
P0340: camshaft position sensor circuit
P0340 means the engine computer is not getting a usable signal from the camshaft position sensor. The usual symptoms are long cranking or an intermittent stall. Causes, what to check first and indicative UK costs. Read more...
P2002: diesel particulate filter efficiency low
P2002 means the diesel particulate filter is not filtering effectively or is clogged with soot, most common on diesels used mainly for short journeys. Regeneration, the MOT position and indicative UK costs. Read more...
P0299: turbocharger underboost
P0299 means the turbocharger is not producing as much boost as the engine computer asked for, usually felt as lost power and limp mode. It is often a leak or a sticking actuator rather than a failed turbo. Read more...
P0171: system too lean (Bank 1)
P0171 means bank 1 is running lean: too much air or too little fuel. Usually unmetered air getting in, or a lazy mass airflow sensor. The fuel trim test that splits the causes, plus indicative UK costs. Read more...
P0128: coolant temperature below thermostat regulating temperature
P0128 means the engine is not reaching its normal running temperature in the time the engine computer expects, nearly always a thermostat stuck open. Causes, what to check first and indicative UK costs. Read more...
P0087: fuel rail pressure too low
P0087 means fuel pressure in the rail is below what the engine computer asked for. It is most common on diesels and usually shows up as lost power or limp mode. Causes, what to check first and indicative UK costs. Read more...
P0016: crankshaft and camshaft correlation (Bank 1)
P0016 means the crankshaft and camshaft sensors disagree about where the engine's timing actually is. On many engines that points to a stretched timing chain rather than a sensor. Causes, what to check first and indicative UK costs. Read more...
What does a car diagnostic cost in the UK?
A car diagnostic in the UK costs roughly £25 to £120. Most sit between £50 and £100. Verified prices from Kwik Fit, National, Halfords, the AA and the RAC, what each fee actually buys, and where a £20 remote read fits. Read more...
P0442, P0455 and P0456: EVAP leaks (small, large, very small)
P0442, P0455 and P0456 are all leaks in the fuel vapour system, at three different sizes. The commonest cause of all three is a fuel cap that is not sealing. Likely causes, what to check first, indicative UK costs and what it means for your MOT. Read more...
P0300: random or multiple cylinder misfire
P0300 means the engine is misfiring across more than one cylinder. A flashing light means stop soon. Likely causes, what to check first, indicative UK costs and what it means for your MOT. Read more...
P0420: catalyst efficiency below threshold (Bank 1)
P0420 means the catalytic converter on bank 1 is not cleaning the exhaust as well as the engine computer expects, or the sensor watching it is misreading. Likely causes, what to check first, indicative UK costs and what it means for your MOT. Read more...