Originally Posted by xintersecty

I have the same issue. I just recently saw a post involving Svengoolie and rebuilt duo-valve. He took the rebuild of the duo-valve to a whole new level compared to my rebuild job. Wait a sec, you are the famous Svengoolie! LOL! Nicely done there. I have three theories as to why there is "hot air". 1) There is a hot/cold deck mixer and a possible vacuum leak causing the door to flip to the hot side. 2) The climate control system is farked and just turns on the duo-valve flooding the cabin with unneeded hot air. 3) Almost like the theory 2, but the coils in the duo-valve are failing.

Does the duo-valve fail open? If it fails close, then just unplug the duo-valve.