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so i have had this problem since i bought the truck if i turn the temp dial to hot it wont go back to cold . To fix it i would have to turn the truck off turn it to cold then it would work but today that stopped working . Has anyone had this problem is it the switch or a vacume valve in the dash ? If anyone can help please let me know i live in AZ and it about to start getting nasty . Thanks
Kinda need to know what kind of truck you have. My guess is 03-05 full size. The 99-02 have the same style system but in single zone not dual. They're not as failure prone but do go out. By 06 the issue was pretty much fixed. The 05's have new HVACM software that helps too. In any case the far and away most common cause is a crapped out electric actuators. There are no vacuum actuators on the GMT800 and 900 body trucks.
Sorry i keep forgetting to list my model its an 01 1500 ext cab. Thanks for the direction
Take the panel off the underside of the dash. In the center over the trans hump there is a small electric motor with an exposed drive. Feel of the drive with your finger while moving the temp knob. If it doesn't move there is a good chance you need a new actuator motor.