Some of these on newer vehicles have electric "actuators" which will "resynchronize" themselves when power is removed, then restored.

With the vehicle off, try removing the fuse or fuses to all the A/C, fan, HVAC, etc. gizmos, wait about 10 minutes, then replace the fuses.

Then start your vehicle and DO NOT touch any buttons in the vehicle for 1 minute. You may hear the actuators moving and resynchronizing themselves.

Then see if it works.

If you must, remove a battery connection for 10 minutes, but that may cause you to lose radio and other settings. Also vehicle emission stored information. You many need to drive for several days at different speeds to get that information back into the emissions computer and then be able to pass a vehicle emissions test. No biggie if you do not need to get an emissions test any time soon.

If that does not solve the problem, then you would need a factory service manual set of books to troublehoot and replace an actuator. Or go to alldatadiy.com for the same info.

 

The diesel version of these trucks use an electric vacuum pump, without vacuum the heater defaults to the defrost for safety. You may have a bad fuse, pump or broken vacuum line. They also use vacuum to operate the locking hubs on 4x4 versions, a leak in parts of that system can rob all the vacuum.

 

The diesel version of these trucks use an electric vacuum pump, without vacuum the heater defaults to the defrost for safety. You may have a bad fuse, pump or broken vacuum line. They also use vacuum to operate the locking hubs on 4x4 versions, a leak in parts of that system can rob all the vacuum.

Agreed. See bad vacuum pumps and split vacuum lines all the time. Or the pump will run but only make 3-4" of vacuum, not enough to move the 'mode' door.

***the defrost position is the 'default' no vacuum door position***

 

Yep most likely vacuum issue. Key on engine off you should hear the vacuum pump running. Located on right fender liner. Common for broken lines too.

 

sounds like the blend door actuator, just repaired mine in a 02 silverado, not bad at all to do.

 

Great suggestions guys! I'm gonna check it all out & report back. Thanks!

 

Had this happen before, vacuum line leak on the SOF 4x4 system. When the front hubs were swapped out for Warns (all three have them) a vacuum line was never properly sealed with a couple years later sprung into a bigger leak which caused the heat and air to only blow out of the defrost. The leak was a simple fix.

Got three of these trucks with a combined miles of close to 800,000.

All have the SOF 4x4 system and all have had issues. To do over again would get the floor activated old fashioned bullet proof 4x4 system. When you go aftermarket hubs you gotta lock them in anyways.

 

If I'm not mistaken, the vacuum pump that runs the HVAC & 4x4 on these trucks is electric, with a plastic housing. It's located on the passenger-side front fender, very close to the cabin fan. It is a common problem, and a fairly easy and inexpensive fix.