sometimes my air cond./heater gets stuck and blows hot air with air cond on? on 2001 GMC Sierra 1500

has only happened twice this july. goes away when engine is shut down and cooled.

Asked by for the 2001 GMC Sierra 1500
same problem on toyota highlander, seems tightening the nut under the selection knob that holds the selector in place, solves the problem....temporarily anyway. guess i'll try some locktite next time to make it more than temporary.