Got a dashboard warning light? Enter your OBD-II fault code below for a plain-English explanation — urgency rating, likely causes and UK repair cost estimates tailored to your vehicle.
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P0300 Random or multiple cylinder misfire
Your engine is misfiring across more than one cylinder. You may notice rough idling, a juddering sensation or a flashing engine warning light. The most common causes are worn spark plugs, a faulty ignition coil, or low fuel pressure. Get it checked promptly — prolonged misfiring can damage the catalytic converter.
P0301 Cylinder 1 misfire detected
Cylinder 1 is misfiring. You'll often feel a noticeable shake or judder, especially at idle or under load. Start with the spark plug and ignition coil on cylinder 1 — these are the most common causes and relatively straightforward to replace.
P0420 Catalyst system efficiency below threshold (Bank 1)
The catalytic converter on bank 1 isn't cleaning exhaust gases efficiently enough. It's common on higher-mileage cars and doesn't usually mean the car is immediately unsafe to drive, but it will likely fail an MOT emissions test if left. A faulty oxygen sensor is sometimes the real cause rather than the converter itself.
P0171 System too lean (Bank 1)
The engine is running lean on bank 1 — there's too much air or not enough fuel in the mix. Common causes include a vacuum leak, a dirty or faulty mass airflow sensor, or weak fuel injectors. You may notice rough idling or hesitation when you accelerate.
P0442 Evaporative emission small leak detected
A small leak has been detected in the fuel vapour system. The most common cause is a loose or worn fuel cap — tighten or replace it and the light may clear itself after a few drive cycles. If it persists, a cracked vapour line or faulty purge valve is the next thing to investigate.
P0128 Coolant temperature below thermostat regulating temperature
Your engine coolant isn't reaching normal operating temperature. The most common cause is a thermostat stuck open. It's a relatively cheap repair and worth doing promptly — running cold affects fuel economy and increases engine wear over time.
P0401 Exhaust gas recirculation flow insufficient
The EGR (exhaust gas recirculation) system isn't flowing enough gas back into the engine. This is particularly common on diesel engines. The EGR valve often gets clogged with carbon deposits over time and can sometimes be cleaned rather than replaced, saving a significant amount on parts.
P0011 Intake camshaft timing over-advanced (Bank 1)
The intake camshaft timing is more advanced than it should be. This is often caused by low or dirty engine oil — the variable valve timing system relies on oil pressure to operate correctly. Check your oil level and condition first. If the oil is fine, a worn timing component or faulty camshaft position sensor may be to blame.
About Motorclue
Motorclue is a free OBD-II fault code explainer for UK drivers. When your car's warning light comes on, the engine management system stores a fault code — a short identifier like P0300 or P0420 — that points to what triggered it. Motorclue takes that code and turns it into plain-English: what the problem is, how urgent it is, the most likely causes, and a realistic UK repair cost estimate tailored to your make and model.
To use it, enter your registration plate to pull your vehicle details automatically, then add the fault code from your OBD reader. No code yet? You can also just describe what you're noticing — a grinding noise when you brake, a judder on acceleration, or a rough idle — and Motorclue will suggest the most likely causes based on your vehicle.
Motorclue is built for any UK driver who's seen a warning light and doesn't know where to start. You don't need mechanical knowledge — just your reg plate, a fault code or a description of the problem, and a few seconds.