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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
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Experience:  GM Grand Master Technician 2007. 14 years experience.
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2001 suburban _ HVAC mode actuator not responding correctly. With

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2001 suburban _ HVAC mode actuator not responding correctly.
With the engine off and acc on, when I rotate the mode actuator, I can hear something like a door rotating or a shaft turning on the driver side behind the vent opening, but all heat is directed to the defroster, always, no matter which location the mode actuator knob is in.
Welcome. The mode door actuator is above the fuel pedal on the drivers side of the heater case. Often these actuators do fail, but if you have the automatic system, it may the control head. I would start by checking the actuator and the sliding linkage. Lube it with a light oil.
Customer: replied 6 years ago.
GM Tech. Yes, I DO have the electronic climate control system. So, I should check the actuator first? I definitely hear some action taking place as I rotate the knob from auto to the face vent position, then again two more times somewhere between there and the defrost position. I'll check the actuator linkage first. I have air tool oil; will it be sufficient for the lubing?

That would be. Try and see. Watch what happens.

#04-01-39-007: Loss of HVAC System Mode and/or Temperature Control (Replace HVAC Control Module) - (Jun 17, 2004)

Subject:Loss Of HVAC System Mode and/or Temperature Control (Replace HVAC Control Module)

Models:2002 Cadillac Escalade, Escalade EXT
1999-2002 Chevrolet Silverado, Suburban, Tahoe
2002 Chevrolet Avalanche
1999-2002 GMC Denali, Sierra, Yukon, Yukon XL
2001 GMC Sierra C3
2002 GMC Sierra Denali
with Automatic HVAC System (RPO C68)




Some customers may comment that they cannot control the HVAC system modes and/or the temperature. These concerns may be intermittent.


An HVAC control module logic lock-up may be the cause of these conditions.


Important: Prior to replacing the HVAC control head, review Corporate Bulletin Number 01-01-39-007 (SI Document ID #845221).

Replace the HVAC control module. Refer to the HVAC Control Module Replacement procedure in the HVAC System -- Automatic sub-section of the Service Manual.

Part Information

Part Number



Cadillac, Celsius Temperature Display (RPO U34)

HVAC Control Assembly (Module)


Cadillac, Minus (-) Celsius Temperature Display (RPO U34)

HVAC Control Assembly (Module)


Chevrolet/GMC, Celsius Temperature Display (RPO U34)

HVAC Control Assembly (Module)


Chevrolet/GMC, Minus (-) Celsius Temperature Display (RPO U34)

HVAC Control Assembly (Module)

Customer: replied 6 years ago.
I located the actuator... which raised another question.
There is a white gear and a black gear. The black gear moves a little but it looks like there is about a 1 inch section with no teeth on it that are maybe supposed to engage the teeth on the white gear, so that the white gear will move the doors? It looks like the teeth are broken off the black gear, or at least worn off. Does this makesense and if so, how do I replace the black gear.
This is getting into the bonus area, eh? Thank you
That is designed that way. IT will only engage the teeth and move it when the bigger black gear moves into position.
Customer: replied 6 years ago.
The bigger gear is not moving enough to get into position to move the white gear. It only moves about 1/4 inch each time and only moves within the area without teeth. Could it have dropped out of position or is this where the control head comes into question?
Sorry for all the questions, but I'd like to narrow the problem down enough to know what needs to be replaced/repaired.
Thanks again
OK, this sounds like either the actuator is not allowing it to move enough or there is something in the case blocking it like a pen or something.
Customer: replied 6 years ago.
Oh, my wife just informed me that this whole problem started immediately after I replaced the battery. Is that just a coincidence or could it be related?
If not, how can the actuator be not allowing it to move enough, or is that the "something in the case" you were referring to? If other than something in the case, is that something I can fix? How do I get into the case to clean out the "something"?
Ok, that means this is the actuator. Try removing the battery again then allow it to recalibrate again. What happens is the feedback circuit in the actuator gets dirty and the system looses the learned position. If this does not work, replace the actuator.
Customer: replied 6 years ago.
Should I disconnect it for a certain amount of time or just on/off? Do I need to do anything to help it recalibrate or will it do it automatically? Can I clean the circuit to help or is that just an "electronic" term?
Thanks for all the help.

Off for 10 seconds will do fine and it will do it automatically when you turn the key on.

This issue is inside the sensor and you cannot clean it. In a sense, the calibration sweep does start to clean it.

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Customer: replied 6 years ago.
Awesome dude. My wife is SO happy, and so am I, obviously.
I am accepting your answer and am going to bookmark you.
Thanks a lot.

Thanks. To get me directly the next time, in the title line put "For BlueGorilla Only" and I will get it as soon as I am able.

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