How can I diagnose a faulty oxygen sensor in my Audi 80?

I recently noticed a decrease in fuel efficiency and a slight decrease in engine performance in my Audi 80. I suspect that the oxygen sensor may be faulty. How can I confirm this and what steps should I take to fix it?

First, you can use an OBD2 scanner to check for any trouble codes related to the oxygen sensor. If any codes appear, it is most likely a faulty oxygen sensor. As for fixing it, you will need to locate the sensor in your Audi 80 and replace it with a new one. Make sure to also clean out any debris or build-up in the sensor's housing.