What causes a 'Check Engine' light on an Audi 80?

I recently bought an Audi 80 and the 'Check Engine' light came on after a few weeks of driving it. My mechanic couldn't find anything wrong, but the light is still on. What could be causing this and how can I resolve it?

The 'Check Engine' light can be triggered by a variety of issues, such as a loose gas cap or a faulty oxygen sensor. It's important to have the car's computer codes read to determine the exact cause.