This means that the engine fuelling, ignition or exhaust system has a fault, and that fault is stored as a code in the car's on board computer. This fault code can be read by any garage with suitable diagnostic eqipment - and it doesn't have to be a main dealer. Once the code has been read, you'll be given advice on what to do next, and the lamp can be turned off once the fault is sorted.
John, April 2009
