I have a 2004 gmc sierra ext. cab 4x4 and air or heat will only come out of defrost. wont blow out vents or feet.
Sounds like there is a vacuum leak, possibly under the hood.. Vaccumm is needed to change the mode doors.
first step is to locate the source of the leak which is the challange. Could be under the dash, or even as far away as under the hood. The actual fix could be as easy as reinserting a loose vacuum hose or as difficult as the replacement of the mode door diaphram which requires in depth knowledge on disassembly
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Ryan I have a 1994 k1500 and it operates off of electronic modules that moves the doors instead of the older models with vacuum operated doors, With this said it might be the controller rather than these individual electric modules. These modules are located on your heater blower box under your dash. I would recommend going and getting yourself a manual to locate the modules on your unit.
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
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how do i fix it?
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