Why does my AC not blow cold air?

I had a 2004 Yukon. My nephew is a mechanic and used to travel with me in it racing and i had the same issue only cool or warm air at an undesired time. If i ran the heater in the morning and just turned it off and it was warm out that day it would still be vented to the warm side and warm air would bleed it. same it was hot and i ran the AC or vent then it got cold out it would bleed in cool air. He explained this and by just switching on the heater or ac and changing it and shutting it off would change what side of the vent it would pull air from.

Thanks, but nothing I do with the adjustments will help this. The air temp stays the same unless I turn the temp up. I kinda know this is about a door not working right for some reason. I was hoping someone had fixed this problem and would let me know how. I keep hoping it will start working by changing the temp adjustment.

Not sure if you got this text but it sounds like you’re actuator door (for temperature) is not working