Check engine light

The engine management system has detected something outside normal limits and stored a diagnostic code. The light says a fault exists; it does not say what the fault is, and the same symbol covers everything from a loose fuel cap to a failing sensor.

• A fuel cap left loose or not sealing
• An oxygen (lambda) sensor past its life
• A misfire from worn spark plugs or ignition coils
• Catalytic converter efficiency below threshold
• A mass airflow or other engine sensor reading incorrectly
• A leak in the emissions or intake system

If the light is steady and the car drives normally, you can usually continue at moderate speed and have the code read soon.

If the light is flashing, or the car is losing power, juddering or making an unfamiliar noise, stop somewhere safe and stop driving. A flashing engine light usually means a misfire, and raw fuel reaching a hot catalytic converter can destroy it within minutes.

Any time the light is on. The stored code needs reading with a scan tool. Guessing at the cause is how people end up replacing parts that were never faulty.