I have a 2005 Yukon that within the last few days has been blowing cold air when the heater is on. If we turn the heater fan off and wait about 5-10 minutes, the heater will start working. Completely dumbfounded, never had an issue like this before. Anybody have any idea what's going on??

by the way the temp is at normal operating range when this is happening

 

there are motors that open the hot/cold air doors and they fail pretty often. They are under the black cover you have to remove to get to the cabin filter. Mine has one but since yours is newer it may have more. Remove the cover and listen to see if they move when you turn the ****.