When we auto Start our 2015 Colorodo to warm it up and defrost the windows, sometimes the blower does not come on until after we put the key in and start it. and if the blower does start it stays cold again until we start the truck and put it in drive-- it has happened several times.. today my wife sat in the truck for over 20 munutes and the blower was on high and did not start blowing warm air until she put the truck in drive.. Must have something to do with the computer.

 

Check your heating blend door.