How do I troubleshoot a heating issue in my Audi A2?

My Audi A2's heating system doesn't seem to be working properly. The car takes a long time to warm up and the interior doesn't feel as warm as it used to. What could be causing this issue and how can I fix it?

First, check the coolant level in your vehicle. A low level could indicate a leak or a problem with the radiator. If that checks out, then it could be an issue with the thermostat. Consider replacing it to see if that improves your heating.