I'v been having exactly the same prob. I found some blown body control fuses, but nothing improved. Its wierd, it works diff at each key in...sometimes runs in "default" only blowing thru the defrost vents, other times only hot, other time hot on pass only, then another it'll work. Of course the dealer didn't have a clue, so I told them to order a new head unit...still waiting...
Hey TORQUE_WRENCH...I aLso have been getting intermittant msgs on the display. First my 4wd was flashing service and got stuck in awd, and now its telling me to service air bags. Every once in a while I get 'engine reduced power' and the TB won't respond...are these systems related...is the a BCM relay I shud be lookin at? Thot I had a ground or loose wire prob, but nothing I cud find so far...suggestions?

 

i ordered a new unit so I'ma see if that fixes it.

 

someone posted a GM TSB about this, it talks about some wires chafing under the dash support, I think it was TURBOGIBBS, I'll see if I can find it!

 

Oh see right here, it might help if the problem is still there after you install the new unit
https://www.performancetrucks.net/fo...d.php?t=422918

 

Well. I knew it was releated to my PCM swap. I got a new unit from another member and when I was putting it in I decided I'd give my unit 1 more chance, so I didn't throw away a good unit.
I unplugged it from the dash for a few seconds and plugged it back in and it started working. That was the only thing I hadn't tried yet.
So now I have a spare.

 

Could possibly be the BCM but sounds more like a PCM related problem. You need to ping both the PCm and BCM to see if they are responsive. But it may likely be cured with a simple re-flash of the both modules.

 

What did your technician say? If it were the Blower Motor Resistor & Connector Harness problem, usually they will blow on one certain fan speed and not the others. The problem with that particular repair is, the Resistor draws too much power for the wiring harness that it's connected to. The wires will in turn melt and cross-over. Then you develop a short or it will over heat the resistor and only one or less fan speeds will operate. But to me it does sound like you have a defective AC Control head.