I was hoping some would could help. With the A/C on or off I cant get heat from the left side. Right side seems to be woring ok.
2005 2500HD with LLY.

Well, I had a problem similar to this. Started like that, then it went to only heat no A/C. If I cycled the ignition it would return back to normal. Then after a few min it would return back to broken. Ended up being an actuater (sp?) Anyway 500 bucks later I was back to normal - yay....