The defrost air only works and I bought a new climate controler and it still blows air only out the defrost. It will blow hot or cold air. I pulled the fuse under the hood and the air started coming out the vents and will not come out the defroster and it is just hot air because when you pull the fuse you disable the air conditioner. It is a 2003 silverado with 4.8 v8

 

Sounds like what happened on my wifes Envoy. It turned out to be the motor that opens and closes to direct the air flow. I repaced it myself. You will have to figure out which motor is not working. I did this my running the air flow controller through all the options until I found the motor that never moved. It is difficult to tell if the motor is moving. A mirror a good flash light and someone to do the switching will help. The only thing that will move is the shaft that is visible but flush with the motors plastic casing. The shaft has a notch in it that will change position as the motor activates. These motor are called actuvators as I remember. Dealer item!