What makes the heating system not switch back and forth from heat to air? on 2002 GMC Yukon

If you have the heat on will not switch to air but if you switch and then shut the engine down for 3 or 4 hours it will switch.

Asked by for the 2002 GMC Yukon
Sounds like a possible temp/blend door actuator. It can be scanned with a professional scan tool to verify the fault.