How do I troubleshoot a malfunctioning air conditioning system in my Audi A4?

I've been experiencing issues with my Audi A4's air conditioning system, where it is either blowing warm air or not working at all. I've checked the fuses and refrigerant levels, but everything seems to be in order. What steps should I take to diagnose and fix this problem?

First, you should check the condenser and compressor for any signs of damage or leaking. If everything looks okay, try cleaning the evaporator and condenser coils to improve airflow. If that doesn't work, it may be a problem with the blend door or control module.