Hi! I'm new so, bear with me. I have a 2004 GMC Sierra 1500. I had only had it a few months when, as I was driving along with the a/c on, the heater came on!?!?

At first, I could take my shoe and hit the "little black box" "thingie" under the dash and above the feet of any passenger and the cold air would return. Another thing I could do was turn the temp control lever from cold to hot several times and the cold air would return. Neither of these things work anymore. It's over 100 degrees in Texas!!!!

It seems as though, when the a/c is working, the heat comes on when I de-accelarate. My husband says this sounds like a vacuum problem to him?

And, yes, I took it to the dealership immediately. Actually, I have had it there 11 times -- Yes, 11!!! -- with this issue. They have replaced the control assembly, the "21997350" control, the blend door actuator, and the right actuator temp door. Other times, they have "ordered parts", been "unable to duplicate problem", or (can you believe??) simply not noted it on the invoice when I was there for other items.

I am at my wits end. I have researched on-line for hours and hours trying to find something. Of course, my warranty is out but, since this was never resolved, I consider any a/c work still covered.

Thank you and any suggestions or information would be ever so appreciated.

 

My 2001 did this a long time ago. When I was driving on the highway it started blowing hot. I messed with all the switches and nothing. I turned it off and back on and it was fine. Like it was tempermental. So when it did it again I popped the hood and looked around and noticed that the accumalator and the lines were all iced up bad. And it was hot outside. Turns out it was the cycle switch. Replaced it and worked fine.

 

Thanks!! I'll check it out. Keep the ideas comin, though, Please?????

 

Your control head is probably to blame. Our 2004 Escalade did the same thing and I could shut the truck off and restart it and it would go back to cold. My fathers 05 did the same thing and he recently replaced the contoller and it has been fine ever since.