jantjies

I own a 2004 Audi A3 TDI and I have an issue with my aircon/fan/heater as when I put it on to demists my windows or use the heater it does not work the bars on the dash will show it goes on but no air comes through. Then there will be days that it work absolutely fine. Is there maybe a fuse that needs to be changed or something as I dont know what to do.