How to diagnose and fix Audi 80 heater problems?

My Audi 80's heater seems to be malfunctioning, as it either blows out only cold air or intermittent heat. What could be causing this issue and how can I fix it?

First, check the coolant levels and make sure they are at the proper level. If they are low, refill them and see if that solves the problem.

Another possible cause could be a faulty thermostat. It may be stuck in the open position, not allowing the engine to warm up properly. In this case, the thermostat would need to be replaced.