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.

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.

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.

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.

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.

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.

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.

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.