Climate control suddenly blows cold air with heat on

[ree]

Twice in the past month, I've been driving along with the heater on (no defrost) and, all of a sudden, the climate control starts blowing really cold air. No amount of futzing with the controls gets it to blow warm. In both cases it started acting normally after stopping and restarting the truck.

Clearly there's something whacked in a computer. Would a code have been thrown for this sort of event to help identify why the climate control felt it needed to blow cold air? If so, how many ignition cycles do I have to get it to a dealer to read the code and diagnose the problem?

Perhaps it's time to order my own programmer.

If no codes are involved, how the hell should I get this diagnosed and resolved. It's a very spurious problem. I'm sure it'll never reproduce at the dealer. And should I worry about the AC compressor cycling when it's sub-freezing when this happens???

[Adam in CO]

Radiator problem. Check the coolant level, for leaks, for radiator or hose blockage. No coolant means nothing flowing to heater core. There wouldn't be any codes.

[ree]

Originally Posted by Adam in CO

Radiator problem. Check the coolant level, for leaks, for radiator or hose blockage. No coolant means nothing flowing to heater core. There wouldn't be any codes.

Rebooting the truck would fix a low radiator level or a blocked hose? I'll check, but it seems to infrequent to be this. Also, the coolant temp looked normal in both cases.

[timgco]

I believe there are TWO plunger switches under the dash that go bad. I have the same intermitent issue as well on my 06.

[BlueHUMMERH2]

Yeah, take it in and have the blower box checked. There may be a blockage or some kind of debris that is keeping the flap from opening/closing. I get leaves into my blower sometimes (I don't know how) and I've also had snow come inside.... But it's probably not anything more than the cold/hot flap thingy getting caught up.

When I had this problem, I was blowing cold on one side and hot on the other, if that helps. That's why I figured the diverting flap was stuck, and wasn't moving when I changed the A/C controls.

[Alan06SUT]

This happened to me when I turned one all the way hot and one all the way cold, then tried to turn them both back to hot, only the 1 hot side stayed hot. The cold side never got hot. The next day everything seemed to work fine??? So, how can you check the flap? I dont want to go all the way to the dealer.

[SnakeH2]

Originally Posted by BlueHUMMERH2

When I had this problem, I was blowing cold on one side and hot on the other....

That's happened to me a couple of times. I shut the whole system down and that seemed to correct it.

[ree]

First time for me too was left only. The time today was both.

I just took a look in the service manuals. Perhaps I'm not looking in the right place, but all the HVAC door replacement procedures (Recirculation, Mode Upper, Mode Lower, Air Temp Door Left and Air Temp Door Right) start with "Remove the HVAC module assembly" which starts with "Drain the engine coolant" and a bunch of other stuff that doesn't look too simple.

I haven't yet searched the manual enough to see if there's a simpler procedure to get at the doors enough to inspect them. If I figure it out, I'll let you all know.

I really seems like a software/firmware bug though or maybe some sensors that indicate the orientation of the doors (wild speculation).

[Goalkeeper]

I also had this issue happen for the first time recently. Instead of hot air for the center right / right, I was getting Cold air!

It hasn't happened since and only that one time....so far.

[HUMTECH]

You can take it to the dealer and they can try an HVAC door relearn procedure with A tech 2. Also you may have a blend door actuator going bad and intermittantly defaulting to the cold position.

[lasvegash2]

i am also having the same issue, i set it to 80 but it blows out cold air every so often. when i turn the iginition off and then start it back up, it works just fine. one other problem, when i have it set to auto and i start the truck up, i get a buzzing sound from behind the dash, almost like actuators sticking or something.