URQ20V makes a good point. I've had many problems with this warning light on my A4. If the car is running fine but the lite stays on, make sure you are running the highest octane you can get. I found in the past that my check engine light would come on if I used a lower octane. It did however reset itself within a few days after running the higher octane. If it stays on, it may indicate another type of problem.
Good luck.
Good luck.