Engine management light on? What to do (and what it costs)

Updated 23 July 2026 · OBDCode UK editorial

Steady vs flashing: the only distinction that matters

The EML (also called the check engine light or MIL) is the engine computer telling you it has logged a fault code. A steady amber light covers hundreds of possible faults — from a loose fuel cap to a tired oxygen sensor. Performance may feel completely normal. A flashing light is reserved for misfires severe enough to dump unburnt fuel into the exhaust, where it burns inside the catalytic converter. Minutes of driving like this can turn a £30 spark plug into a £1,000+ catalyst replacement.

Will it fail the MOT?

Yes. Since May 2018, an illuminated engine management light is a Major defect — an automatic fail, even if the car drives perfectly. Testers also check that the light works: if it doesn’t illuminate at all during the bulb check, that’s a fail too. If your MOT is coming up, deal with the light first; see what else commonly fails on your model.

The 10-minute DIY diagnosis

1. Read the code. Any OBD2 scanner plugs into the socket (usually under the dash, driver’s side) and shows a code like P0420 or P0301 in seconds.
2. Look it up. Our fault code library explains every standard code: severity, causes, DIY steps and realistic UK repair costs.
3. Check the cheap stuff first. Loose fuel cap (EVAP codes), disconnected vacuum pipe, corroded battery terminals — a surprising share of lights are fixed for free.
4. Clear and monitor. If the fix worked, the code stays away after a full drive cycle. If it returns, the code page tells you the next step up the ladder.

What a garage will charge

A scanner pays for itself the first time you use it — and lets you verify the garage’s story. See which scanner to buy.

General guidance for UK drivers, not professional advice. MOT rules follow the DVSA inspection manual (2026 edition).