Your check engine light just started flashing — and your heart sinks. A flashing light is not the same as a solid one, and it demands immediate attention. This guide explains exactly what's happening, whether you can keep driving, and what to do next.
In the UK, the check engine light (also called the engine management light or EML) can behave in three ways, each with a different meaning:
In almost every case, a flashing check engine light means the engine is misfiring — one or more cylinders are not firing correctly. When this happens, unburnt fuel passes through the exhaust and into the catalytic converter. The catalytic converter runs at around 600–800°C under normal conditions; with unburnt fuel, it can exceed 1,000°C and melt internally. A new catalytic converter costs £200–£800 for most UK cars. A set of spark plugs costs £20–£50. The cost of acting quickly is dramatically lower than the cost of waiting.
Common causes of a misfiring engine include:
No. You should reduce your speed immediately, avoid hard acceleration or high RPM, and drive to the nearest garage or get the car recovered. If the engine is running roughly, juddering, or you can smell unburnt fuel, pull over as soon as it's safe to do so and switch off the engine.
If you scan the car with an OBD2 scanner, you'll typically find one or more of these codes:
The specific cylinder code (P0301–P0306) is the most useful — it tells you exactly which cylinder is misfiring, which points directly to the coil, plug or injector for that cylinder.
Yes — in two ways. First, any illuminated check engine or engine management light is an automatic MOT failure in the UK regardless of what it means. Second, active misfires produce high hydrocarbon emissions that will fail the emissions test independently. Fix the misfire first, then check the light is off before booking your MOT.
| Cause | DIY Cost | Garage Cost |
|---|---|---|
| Spark plugs (full set — see P0300 misfire guide) | £20–£50 | £60–£150 |
| Ignition coil (single) | £20–£60 | £80–£180 |
| Fuel injector clean | £10 (cleaner) | £80–£150 |
| Catalytic converter (if damaged) | N/A | £200–£800 |
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