Heater Blower/Fan

[GMCDenali]

I am having an issue with my '05 Yukon Denali XL's heater blower. The core is making heat, however, the fan sporadically will not come on for long periods of time. The fan for the rear seats (both upper and lower) seems to be working fine and comes on/adjusts intensity with the controls. I have tried many things from restarting the vehicle, to shutting the heat/AC control panel off. Is this some sort of thermostatic control issue? It seems to happen mainly on days that are colder than normal. When everything works, it works great and heats the vehicle very well.
Edit/Delete Message

 

[Noto81]

I just had the control module repalce in my 03 Yukon because of similiar issues. The blower wouldn't adjust speed no matter if it was on hi, low, or somewhere in between. The other night the fan stayed on after i shut my truck off.