Blend door malfunction. check for a broken door, or bad door actuator.
SOURCE: 1999 gmc safari van upper vent
Check under the hood for deteriorated small vacuum hose coming off the intake over to the heater box area.
Repair the vacuum hose.
SOURCE: a/c and heater only blow air out of defrost
either a vacumn pipe is broken or the ends split so its not passing a vacumn to open the flaps,where they are and how to get to them i cant help because i have never worked on one as they are not here in spain.check the engine end first though .Look for some small rubber pipes running from the inlet manifold
SOURCE: I replaced heater core in 1999 gmc sierra 1500 5.3
no,, the system works from a vacuum and has to be worked on in a service garage. sorry about the bad news...
SOURCE: 2004 gmc dual ac, driver side vents shift from ac
bad blend door actuator maybe there is way to press the buttons on controller to make doors recenter they call it
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