What causes the infamous 'check engine' light in an Audi 80?

I recently had to replace the catalytic converter on my Audi 80 and now the 'check engine' light keeps coming on. What could be causing this and is there anything I can do to fix it?

The 'check engine' light can be caused by a variety of issues, such as a loose gas cap, faulty sensors, or a problematic catalytic converter. It may be worth checking and tightening your gas cap first before seeking further assistance.