dvs_dave

As said, make sure it's bled properly. If that doesn't help, then it's time for a VCDS scan, see what it says

MP4.2+6.0

One more big picture thought... is basic thermostat confirmed good?? Start with available temp. gauge information even if you have no VCDS. Is temp guage in middle of display range like it should be normally, or is it typically lower on cold days like only ⅓? Not precise way to diagnose, but if reading low like that on gauge your thermostat may be stuck partially open and it is running too cold. On a 2004, I would increase the odds the thermostat could be no good by now if not previously changed like for a 4.2 (gas) timing belt full service.

jlbell

If you would like to know more information about the climate system operation and functions, check out this document: https://www.dropbox.com/s/kw25vf0wpj...irCon.pdf?dl=0

sullyj

Responding to MP4.2+6.0

My car is a 2008 A8L w/4.2. I had a used heater control valve installed but it was leaking so I have a replacement in route. I found a used temp control unit for the dash. The tech said my old one threw a fault code so he feels it needed replacement. My gut tells me this isn't the culprit but I have my fingers crossed. Tired of throwing parts at the car.

MP4.2+6.0

Thanks for update. Again, what about engine thermostat functioning though? See prior replies about it.

Also, check if front dash HVAC control unit has component protection. Trip to dealer if it does for installing a used unit. Rear one does not from having changed that myself on a 4 zone set up, but I don't recall on front one.