hi have a 54 plate a4 2.0 tdi air con works fine but when i put it on to hot or hi it blows cool air its warm for about 5 seconds then goes cold , temp gage stays at 90 noticed fan on all the time, took fan fuse out today still ran at 90 and heating did not get any hotter , could it be air in the system or something other not losing water or anything like that

 

so you have enough coolant? does it only blow hot air while moving and cool air when stopped?

 

Could be the motor to the hot/cold mixing motor. My Jetta GLX's mixing motor and vent diverting motors went bad sending a wrong signal to the main unit, confusing it. When that happened, the temp of the air would rapidly swing, coming out of the vents, as well as which vents it was coming out of.