There are a fairly wide range of things that could be wrong. They include the actuator, control panel and all of the wiring in between. GM issued  TSB’s years ago that referred to loss of logic within the control panel and issues with a harness possibly being misrouted that causes wiring / connectivity concerns.

On the logic loss concern GM had stated that the control panel will need to be replaced. However you may want to try this process instead. Assuming you only have one battery, disconnect both battery cables from the battery and hold the cable ends together for one full minute to clear all learned memory. Make sure you have solid metal to metal contact. Reconnect the battery cables and proceed as if you have already started a recalibration procedure.

Start the engine and Do Not Touch anything.

Allow the engine to run for at least one full minute.

Turn the ignition off for at least ten seconds but no longer than thirty seconds.

Restart the engine and check for proper HVAC operation.

In the process of  clearing logic lock all learned memory will be cleared and other issues may arise. Radio settings will be lost, idle speed may be incorrect and associated with a stalling condition and the transmission shift points could be softer or firmer than you are used to. 

If this does no good and you want to pursue inspecting the mentioned harness routing issue just let me know and I will gather more info for you.