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