Just bought my 2003 Audi A6 4.2 V8 Quattro but found that the heater doesnt work at all.
If the interior fan is on, then the A/C is on and only very cold air comes out.
There is no effect by changing the temp settings, fan settings or A/C on or off.
It appears that water is not circulating through the interior heater, even though the motor gets up to correct temp quickly.
I have bled the air from the cooling system, and have reversed flushed the interior heater. It is not clogged.
Any ideas for the cause ?

 

Did you ever figure this out?? I'm having the same problem with my 2003 a6 3.0.... It'll be hot for a brief second and then the ac kicks on.

 

Yes, I found out it was 2 separate problems.
1. Heater hose was twisted and so no coolant was flowing through the heater.
2. Fresh air blower motor was worn and so worked intermittently. I removed the motor and cleaned the carbon brushes.
Now all works fine.

 

Awesome!! I'll give that last one a shot and see what happens!