you more than likely have a blend door issue. See if you can pull any codes from the GEM and work off of there. If not...get ready to pull the dash apart unless it's the head unit/control unit
-Drew
EDIT: do you throw pens, paper, etc up on your dash? If you do, a pen or something simple can slip down into the vents and get in the way of the doors which will cause your issue. Had a silverador with the same issue last week.