Engine warning light is on: what it can mean and what to do

The yellow engine-shaped light (the emissions warning, often called "check engine") is probably the most common reason for questions like "can I still drive to a garage?". The answer depends on how it came on and how the car behaves.

In general:

• It stays on and the car drives normally. The control unit has logged a fault related to the engine or emissions. It may be something minor (a loose fuel cap, a faulty sensor), but also something that will get worse. Have the fault memory read with diagnostics as soon as possible.
• It flashes. It usually points to misfiring, which can damage the catalytic converter. Slow down, avoid high revs and have the car looked at straight away.
• It is on and the car has no power. The engine may have gone into limp mode to protect itself. Drive carefully to a garage, or call a tow.
• Red warning lights (oil pressure, coolant temperature, charging, brakes) are a different case. With them, stop as soon as it is safe and switch the engine off.

A fault code (for example in the form P0420) tells you which part of the system the control unit is monitoring, but not always the exact cause. Cheap OBD plug-in readers will show the code, diagnostics at a garage find out more.

When buying a used car, ask to see the light come on when you turn the ignition on and go out after starting. If it doesn't come on at all, it may have been disabled or disconnected to hide a fault. A light that is on in an advertised car is a reason to want diagnostics before you buy, not after. More in the pre-purchase checklist.

Is your light on? Write the make, model, engine, year, the fault code if you know it, and how the car behaves. Other owners of the same engine often know what it tends to be.