heat not working

There are little motors that control the airflow to the various vents in the dashboard. If one of these motors fails, or the motor is not being controlled properly (control module or wiring) then you will have issues like this. This will require diagnosing the climate control system to figure out why this is occurring, you'll need a high quality manual or take it to a shop. Manual:http://bit.ly/mitchell_repair_manuals_online Shop: http://repairpal.com/gmc-repair-in-new-lebanon-new-york Good Luck!