Had a problem with the heating in my a4, has been in the garage for days. He has checked for any air locks, changed the thermostat, changed the waterpump(after speaking to the Audi techs). But still has not solved the problem. Only one of the two pipes going into the heater matrix/rad is not getting hot. Any idea what can cause this?? As my mechanic says they should both be hot, he spoke to the Audi techs again and they had no ideas. He's nxt suggestion is to change the radiator???