Thank you all posters, without this forum I would not have been able to do this.
I have a 2006 GMC Envoy Denali. I could not get any air (hot or cold) from the dash vents, only air blowing from the defrost vents and foot vents, and the back seat vents. After hours of searching I could not find my exact car help, but it sounds like this was the mode door actuator, and the dealer wanted $875 to replace this, the dealer said they probably would have to remove the dash. I decided to try this myself. I am not sure if this is the best way to to do this, but I could find no other way on the internet (granted I did not read every single post on here, I figured reading posts for several hours and not finding anything that seemed perfect was enough research).
I was not sure which actuator this was, and I didn't find any good pictures or manuals of where they all are located, so this was a little bit of an exploratory process for me. I started pulling apart the dash, thinking how hard can this really be.
Well, after a few hours I finally had a bunch of panels and part and the dash loose, but I could not get it over the steer wheel. I could not find good instruction on steering wheel removal for my car, until I found this site by accident
Wood Grain Steering Wheel Manufacturer Accessory Chevy GMC OEM Silverado Tahoe Avalanche Suburban Sierra Yukon H2 Hummer in Burl Walnut Rosewood Black Cherry (for steering wheel upgrade kits), this webiste had great instruction on steering wheel removal and help me find out I need a special GM Puller (it has shafts with hooks instead of the normal threaded bolts), that most of the auto parts places did not have, or the workers did not know I really needed a special puller. Once the dash was off, I finally was able to see (and feel the actuator actually making the noise) the mode door actuator was way up in the dash on the drivers side of the center venting stack. I still could not get to the actuator (and see and barely feel it) and thought I would have to remove some of the major metal and cast structure. I decide to try and remove some of the vent ducting that was on top, and as I did, I discovered the large top vent cowl came out rather easily and revealed a relatively easy way to get to the actuator.
Once I removed the actuator (on the white label was HL 52402610 010052792A) the internal vent door swung open (this internal door was not very visible with the large venting cowl removed, revealing that I was on the right tract. I got a new actuator from a local store (Carquest had an aftermarket one in stock) and installed it. I wasn't sure about the position of the acuator and where the vent door should be so I decide to just give it a try and install it. I tested the vent system with everything still exposed and realized the actuator was not fully opening. So I removed the actuator (after disconnecting the battery again) and was actually able to reposition the actuator a little, moving it VERY slowly, and reinstalled it. Now the door would not shut fully, but at least I believe the actuator was at the end of its travel after being moved by the system. So I pull the actuator off again and reinstalled it while holding the internal vent doors in the closed position, and now this did the trick.
BTW, the bolt heads on the screws that hold that actuator would only fit in my 5.5 mm socket (I did not have standard socket that small), and I had to use a little thumbwheel ratchet since the access is still VERY tight to the actuator bolts when coming in from the top.
I reinstalled the dash, but was left with one yellow connetor that I could not find a mate for under the glovebox (see picture which is from a different car I found on this forum but looks the same), if anyone knows what this should be connector to, or it was not connected to start with (I cannot remember when I disassembled this area and I did not document my steps very well) I would be greatly appreciative. Everything seems to work fine now, at least that I have checked. So perhaps this was just not connected to start...?
