How do I fix an HVAC mode selector knob that won't turn. It is stuck on the vent setting and cannot be turned to any other setting. It is stuck.

that is HVAC failure.
the Recirculate mode fresher are mode actuator is dead.
the service manual covers this.
the checks are simple.
first is gain access as stated in the manual.
then the recirc mode motor actuator is seen being dead
as you run the recirc knob off. (from fresh to recirc)
then I take off the actuator and turn the mode damper shaft by hand if stuck bingo if is stuck it try to force it unstuck, cycled 10 times by hand can fix it.
or is packed up in nests inside or leaves. (pull blower and clean out all that junk there)
the if not stuck the actuator motor is dead.
or controls to it bad
read the manual

https://eautorepair.net/Marketing/Default.asp

This is caused by a low engine vacuum, the engine vacuum is low when you accelerate, the HVAC system loses vacuum to the mode control vacuum switch and defaults to the defrost ducts for safety. You likely have a leaking mode control vacuum switch in the control panel of the heater AC system (HVAC).

Fan motors in cars fail commonly. They wear out the brushes and the motor stops or the fuse blows. The other issue with the mode selector may not have anything to do with the motor. The actuator on the mode selector can become stuck or broken and then the air delivery wont go to the right area. All these things should be visible under the dash or behind the glovebox.

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Oct 13, 2011 - 30 posts - ‎21 authors
It will not switch into upper vent mode, only have the option to have it all on my ...You mean the switch itself on the dash will not turn physically?

Turning vent knob (defrost, mix, dash,floor,etc.) has no - Fixya

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Turning vent knob (defrost, mix, dash,floor,etc.) has no effect. Posted by Jim Long on Jan 01, 2016. Want Answer 1. Clicking this will make more experts see the ...

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3) Mode of operation knob has no effect (air comes only to floor for example) 4) Rear defog ... The transistor is MPSA56 (bipolar pnp) and you can locate the faulty one easily. It is the .... With the switch turned only to Vent, no air from anywhere?

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16 year old HVAC dash controllers on GM cars have a bad habit of that. They are 90% plastic and just don't hold up that well. Is it a manual or automatic HVAC system? If it's a manual system, there may be a frozen cable under the dash, but if it's an automatic system, the dash control unit needs to be replaced

The heat/vent blend door that is located inside of the HVAC system behind dash..Rarely it is the control however it could be....Some of the vent door issues are recalls on certain vehicles."Also Blend vent door actuators that operate the doors can become inoperable...If it is the door inside the HVAC it will probably take about 8 hours to repair...Most vent doors either stick shut and or break..as well as actuators....Only a Dealer Scanner can tell exactly where the problem is.....

The binding up is normal for 4x4 mode because the parts which normally are bypassed in a 2 wheel mode are locked and rigid in a 4x4 mode. But being stuck in 4x4 mode is not normal.

Some of the units are Vacuum operated, others are electric. The electric units use a single solenoid with a variable length
plunger that gets its signal from the dash knob. When the lightbulb in the dash selector knob burns out; the voltage changes at the solenoid plunger. The plunger then operates differently than how it is set on the knob selector.

For the electric models, you would put a voltmeter on the wire to the 4x4 shift solenoid and have someone move the selector knob to each position and see if the voltage changes.

THe blend door seems to be stuck. When you change settings from defrost to floor air or, vent, the blend door opens and closes directing the air to the place you want it to go to. I have had some success spraying a lubricating oil such as WD40 under the dash on the blend door. You need to get under the dash with a flashlight to locate it. It looks like a flap. Spray it with a couple of shots of the WD40. Go easy on the spraying as you may be inhaling the fumes. Keep the doors and windows open to minimize the fumes. Wait about 1/2 hour and try the A/C functions. (defrost, floor, vent air)

its a real easy one to fix i had the same problem you just need to take your radio out and then look down on the PLASTIC parts just need to put it back on track
good luck

You need to check and see if the control has vacuum, this is what controls where the air is sent, the switch is called the HVAC mode control selector.