How to diagnose engine problems in an Audi 80?

My Audi 80 seems to be running rough and I'm not sure what the problem could be. How can I diagnose engine issues?

First, check for any stored trouble codes with a diagnostic tool. This will give you a starting point for troubleshooting.

Next, inspect all spark plugs and spark plug wires for any signs of wear or damage.

Also, check the air filter and clean or replace if necessary. A dirty air filter can affect engine performance.