How to fix the air delivery problem? on 2003 GMC Sierra 1500

air only comes out main vents, no defrost or floor when selected. 2003 1500 SLT.

Asked by for the 2003 GMC Sierra 1500
Sounds like a damper door or the motor to control it is not working. you will have to look under the dashboard along the heater box and locate the motor to see if it is moving the arm door or not. Then go from there check voltage is getting to the motor if it isn't moving. If it is getting voltage but not working replace the motor, if it is working and the arm is moving but the door has no change you will need to replace the damper door.