How to troubleshoot low power output in an Audi A4 Quattro?

I have an Audi A4 Quattro that seems to be not performing as well as it used to. The power output seems to be lower, especially when accelerating. What could be causing this issue and how can I troubleshoot it?

The most common cause for a decrease in power output in newer Audi models is a clogged fuel filter. We recommend checking and replacing the fuel filter if necessary. Another possible cause could be a faulty spark plug or oxygen sensor. These can also be easily replaced to improve engine performance.