What is the engine management light?

Three states, three responses

• Amber, steady A fault has been stored. The car will usually drive normally, sometimes in a "limp" mode with reduced power. Get it checked in the next few days; don't ignore it for months.
• Amber, flashing An active misfire is happening right now, and unburnt fuel is going into the catalytic converter, which can be destroyed in minutes. Pull over when safe, switch off, and don't drive it further.
• Red Rare on the engine light itself, but a red engine or oil-pressure warning means stop immediately.

What it usually is

In rough order of how often we see them: oxygen (lambda) sensors, ignition coils and spark plugs, EGR valves, DPF problems on diesels, mass airflow sensors, loose or faulty fuel caps and evaporative leaks, thermostats and coolant temperature sensors, and — increasingly — turbo boost faults. Most are a few hundred pounds or less; none get cheaper by being ignored.

Why it's an MOT fail

Since May 2018, an engine management light that stays on is a major defect: automatic fail. The same goes for ABS, airbag and electronic parking-brake lights. The tester doesn't need to know why it's on; the fact that it is on fails the car.

What a diagnostic actually involves

Anyone with a £20 reader can pull the fault code. That's the start of a diagnosis, not the end — a code says which circuit reported a problem, not which part has failed. A proper diagnostic reads the codes, looks at the live data, and then tests the component before replacing anything. That's the difference between fixing the fault and fitting three sensors until the light goes off.

We charge diagnostics at £48 per hour including VAT, most take under an hour, and you get the findings and a photographed quote before any repair.

Don't just clear it

Clearing the code hides the symptom. The fault is still there, the light will come back, and if it was a misfire you may have damaged the cat in the meantime.