I searched online and found a few instances of this but no solutions. One thread in your Yukon forums (here: http://www.gmtruckclub.com/forum/showthread.php/73938-Heat-while-on-a-c) had multiple posts but also no solutions. I figured I'd bring it here for a broader audience. With the AC on my 2004 GMC Sierra started blowing hot air on the passenger side while the driver's side blows nice cold air as it should. I read up on this, online, and learned that sometimes disconnecting the battery for 10 mins helps. I did this but it made no difference. I'm pretty convinced a door/damper is stuck in a position it shouldn't be ... but am not sure where to look for the door/damper that'd impact only the vents on the passenger side of the cab. Can anyone shed some light on what my next steps should be to troubleshoot and fixing this? Summer's not quite here, yet ... but I need to nail this one before it gets hot, outside.