2000 Blazer LS 4X4. 186k miles. It would seem that my heater control will only work for defrost and face-vent operation. (Just happened yesterday) When I move the controller from the off position to defrost, I can hear the system moving the flaps into position. But it no longer moves for defrost/floor or just floor control. Would changing the heater control solve this problem, or could it be a bad vacuum line? Suggestions? Ideas please?
Vacuum leak or bad actuator. Here is a video to help. http://www.youtube.com/watch?v=Nlt9A1sp-4U
Thank you @knfenimore . I’ll have to try this. Just going to fun since we aren’t expected to get above 22 degrees all week here. 
Sorry that you’ll be in the cold. There is another video showing the replacement of the actuator.
https://www.google.com/url?sa=t&source=web&rct=j&ei=j8LjVJ79NtijyATPjYGIBQ&url=http://www.youtube.com/watch%3Fv%3DWiwZeJlAQdo&ved=0CCsQtwIwBQ&usg=AFQjCNFixm0_L-XLiHJkny-_CkQf3a3KNQ&sig2=4q2C9xyHP8DI4gI9mLZt9g
Thank you AGAIN @knfenimore ! Great video. I even have a reason to buy more tools. Lol. I’m sure I can convince a friend to let me use their garage to change it out. I will definitely be visiting my local junk yard for the part as I prefer to recycle when possible. You have been very helpful. Thank you!
Glad to help. Let us know what you find. Just remember, used parts are just that, used. They can already be bad before you pull them.
22F and it’s stuck on defrost?? Wow, you must be hurting real bad. You better stay home.
At least if it is stuck on defrost you can see, it it was stuck on floor you wouldnt be able to see out the windows unless you turned off the heat entirely, scraped the inside of the windows and opened a window partway to stop your breath from steaming them up.
That is what we used to have to do with tractor trailers in the 60s and 70s when going to Watertown or Montreal.
Just curious, on this vehicle, are these vacuum actuators or some kind of electric motor actuators?
vacuum
At 16 years old and near 200k miles a vacuum leak is possible anywhere.
Blue Blazer
We have a few blazers in our fleet. I always print out the vacuum schematic before I start my diagnosis
In case you need to access the slave mode actuator, you’ll need to lower the glove box door. The service manual will not show you where it’s at
If you do find out exactly which actuator is the problem, gmpartsdirect.com has pretty good pictures, and lists the correct part numbers. The good news is they also use the exact same terminology as in that schematic
Rockauto also has part numbers, but I’m not sure the descriptions are completely accurate.
A simple vacuum pump and a vacuum gauge will also help in your diagnosis. Those tees that hold golf balls also work great for plugging vacuum hoses, for purposes of diagnosis
Thank you @db4690 That schematic will definitely help! I’m just hoping I don’t need to rip the whole dash apart to get it done. Thankfully, I don’t travel too much right now. Warmer weather is on it’s way (40’s), so I may be able to start problem solving soon!