The lamp lights up when the ECU has sensed a problem. Under your steering column, there's an ECU diagnostic port. You can get tools that tell you what the problem is for around £30. They either work independently or hook up to your PC and don't harm the car.
Quite often it's a faulty sensor - if that's the case (on an old car) it may be most cost effective to use the diagnostic tool to simply clear the fault and turn off the ECU lamp. If not, then it'll tell you what's wrong. My light last came on because the transponder (fob) was faulty - had nothing to do with the engine!
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