2004 GMC 3500 Duramax with auto Climate control. Ran A/C on way to track yesterday, no problems, on way home drivers side blowing hot, passenger side blows cold, both knobs on 60°. Is it the main control in center of dash? Or is there another control for the doors? Thanks in advance!

Drivers side blend door actuator is bad more than likely.

Google it.

Driver side is the easy one to change. Bottom of heater case over the trans. hump.

Just did my 03 while I was doing the heater core. Had continuous clicking noise on cold air, right side control. Might be easier to remove dash pad for upper and left center lower controller. I replaced both because I didn’t want to do it twice..Make sure you put the correct part number controller in the correct box...

Thanks for the help guys, I found a way to reset the system on line. Once that was done it started to blow cold on driver side for about 10 seconds, then moved back to heat. Just ordered drivers side blend door by the part# on the old one. This is the easy one, I will let you know the results!

Please update when you change it. My '05 has had this same problem with both sides intermittently forever. The online reset method sort of works sometimes. It's an aggravating issue because it normally happens when it's hot.

UPDATE: Thanks guys, drivers side blend door actuator solved problem! NOTE: bought a cheap one from Amazon for $25, JUNK!!! The connection plug was not cast right, plug would not line up! returned it and got an AC DELCO.

I have been following this as my 05 trailblazer is doing the same thing. I removed the actuator from the driver side blend door and left it hooked up to the harness but just hanging.Double checked to make sure door was closing completely, I still have warm air blowing out the driver side vents and cold out of passenger side. Any other ideas about what could be happening?