Gmc Sierra 1500 hd A/C is only blowing from the defrost and floor. Still blows Hot and cold. But only from Defrost and floor. How to fix??

SOURCE: Heat wont blow through heater or defroster vents

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

SOURCE: a/c and heater only blow air out of defrost

sounds like there is a vacuum leak, possibly under the hood.. Vaccumm is needed to change the mode doors.

SOURCE: the defrost blows air of vents rest of the controls do not blow out any air cold or hot

Does it have auto climate control or dial type HVAC Controls??

SOURCE: 2000 gmc sierra 1500 5.3 v8. a/c heater control problem

your heat/cool and vent selection options are controlled by engine vacuum through small clear tubes attached to the rear of your climate control interface. Other vehicles have small steel pull wires that operate the desired changes. You might try removing the trim and any fasteners affiliated with the interface/console. GENTLY pull outward and look to see if any of these tubes are pulled off or cut/damaged. Did anyone recently install a new stereo in the vehicle? If so, the vacuum tubing and or pull wires that change your outlets could have been damaged when the stereo unit was pushed into place. Hope this helps

Testimonial: "Very good information. THANKS!"

SOURCE: 2005 GMC Sierra 1500 - heater blows, but only cold air

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

Testimonial: "You were absolutely correct. Low coolant level. Put in some 50/50 and we're nice and toasty now, but need to find out why coolant was so low. Thanks."