I have a 2001 S4 and the heat won't work. I noticed also that whenever I turn the heat on it omits a smell of something burning. A mechanic has told me that it needs a thermostat and the bumper has to be removed in order to install it? Does that sound right? I was thinking it was a heater core. What does everyone think?

 

My thought would be the heater core as well. If the thermostat was broken your car would either be running way too hot or way too cold

 

check to see if your cars opertating temperature takes a long time to heat up. if so, then your thermostat is stuck open. your car would overheat if the thermostat was stuck closed.
heater core could be leaking if your getting a smell from the vents. does it smell like antifreeze?
the heater core could be clogged up... mine was this winter, no heat. took it to a radiator shop, and they were able to blow out the crap in the core, and now it works great! that would be your cheapest bet. otherwise... all the other things could be a bit more expensive.

 

i think i am having the same problem with my 2004 A4 the blower on the window works and one of the vents on the drivers side but even on HI it seems colder then it should be. Think its the same problem?