If no change takes place when you select different functions, that is typically caused by break in the vacuum hose feeding the control assembly. The systems are spring-loaded to go to defrost mode for safety. Your feet might freeze but your windshield will be clear.
If you have an electronic controller that provides the different temperatures on the two sides and all different percentages of air flow all over, those gimmicks cause a lot of trouble. The computers are high-failure items as are the actuators. Usually no one wants to work on them so you're tied to the dealer. The computers also have to be programmed to the car through an internet connection, and GM is one of the few manufacturers that selfishly refuse to allow independent shops to do that.
Start by looking on the emissions sticker under the hood. That will show the vacuum hose routing and will show the hose with a small check valve going to the firewall if that is what's used. Check that hose for cracks or if it's disconnected.
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Tuesday, April 30th, 2013 AT 8:08 PM
