My wonderful 4 day old 2001 Audi A4 1.8T Quattro displayed a check engine light 1 day into owning the car. Took it to my mechanic yesterday, he tested it and said the light when on for no apparent reason and there was nothing wrong. He reset it and 4 hours later the light went on again. What could be possible reasons why the light went on? There is no unusual noise, the car runs fine (oil changed yesterday and there was no sludge buildup).