What it usually is
A failing alternator, a broken or slipping drive belt, or a loose or corroded connection.
Check the belt
If the drive belt has snapped, the water pump and power steering may also stop on some engines. Watch the temperature gauge.
Lights dimming
Dimming lights, flickering dash or electrical faults alongside the light mean the battery is running down.
What to tell the mechanic
When it came on, any squeal or noise from the engine, dimming lights, and the age of the battery.
Safe to drive?
Turn off non-essential electrics and drive only a short distance to safety. If the temperature rises or the steering goes heavy, stop. A mobile auto electrician can test it on site.
General guidance only, not a diagnosis. If in doubt, do not drive it and have it inspected. In an emergency, call 111.
Who can help
Get quotes for auto electrician near you
Questions
Will a new battery fix it?
Usually not. If the alternator is not charging, a new battery will go flat too. Ask for a charging test.
What should I put in the job?
The warning light, any noises, and whether the car still starts.