I have been having this intermittent problem with the heater in my 96 A6 2.8. It started out that the heater would blow cold air only, now it is blowing only HOT air regardless of the temperature setting. Even selecting the a/c it still blows hot. Since it is winter this can be ok for awhile, but......not for long.
When it was blowing cold, I back flushed the heater core, but it had good flow and did not help the problem. I even had an extra heater and a/c control display unit (which I swapped out because the origional had the missing pixel problem) but still had the same problem. Is there a temperature control module or sensor that could be acting up and if so where would they be located? the temperature damper seems to be working since it went from cold to hot. I just know its going to blow cold air on the coldest day of the year