Haven't looked yet, but wondering if anyone has had this problem before now.
The passenger side is blowing heat when the driver side has vent or a/c.
Changing the air to extreme cold just shuts down any flow on the pass. side.
So, it's either heat or nothing. Blocking the drivers' vents and cranking up the fan
does get some bleed through, but the air flow is very low.
It would probably be easier if it is just a blend door actuator. I am hoping
it is not the dual climate control itself.

Marty wrote:

Haven't looked yet, but wondering if anyone has had this problem before now.
The passenger side is blowing heat when the driver side has vent or a/c.
Changing the air to extreme cold just shuts down any flow on the pass. side.
So, it's either heat or nothing. Blocking the drivers' vents and cranking up the fan
does get some bleed through, but the air flow is very low.
It would probably be easier if it is just a blend door actuator. I am hoping
it is not the dual climate control itself.

You can see the motor move the shaft while installed to verify its working correctly when hot/cold is turned
I just had a similar issue with mine (the blend door motor) had a cracked gear and would get hung up and stay on heat all the time......
That thing was BEEEOCH TO UNSCREW on the passenger side screw on the GMT400 TRUCKS

Honestly, I'm not sure where to look under the dash for the motor shaft. I haven't picked up a set of manuals for this truck. Do I need to remove the covers? It's one of those problems that doesn't get fixed in a hurry because it doesn't affect me directly; since I am the one driving.

The problem seems to be isolated to changing the temperature control from a warm setting ( like 76°) to a lesser setting ( ~72°). The right side vent stays at the warm setting, while the drivers' side gets cooler. Same is true for tripping on the a/c cooling; the passenger side stays noticeably warmer. IF the truck is turned off and restarted, then the air temperature out of each side appears to be at the correct temperature setting. Warm - if set at warm, and Cool - if set at cool. IF the control is set to a warmer temperature, then the air out of both vents will warm. Going back down in temp is the problem.

Well, I had the dealer put in a new blend door actuator / motor a few weeks ago. Seemed to work fine. Now it is not working.

Now, there is a door clapping / flapping noise ( sometimes constant, other times not ) on the right side. Then over a week ago, my drivers' side a/c went from 74° to full heat. Next time out, normal operation. Today, I am not getting much of any heat on the drivers' side; okay on the passenger side. Sometimes, cycling the mode from heat to defog to vent clears it back to normal; but not always.

I was wondering if it could even be a bad temperature sensor on the drivers' side. But that doesn't cure the passenger side door clapping.

What a pain.

It's also possible that you have a defective control unit. My own 2000 Silverado's AC unit does weird stuff like that, and fiddling with the controls will correct it. Getting a new one is on the list of things to do.

Today was a good example...
Set the ventilation to floor vent with 78° and manual fan. Heat came out the passenger side, but cool air on drivers side.
Stopped at a light, and heard the a/c condensor cycle without the settings being changed. Drivers side started blowing cold air; passenger side hot.
Shut the whole unit off, then heard the blend door motor doing its 'bop-bop-bop' thing. Started driving, and the blowing vent air must have stopped the door from flapping closed.
Stopped at a store, went in / came out, and the unit worked fine til I arrived at home and shut it off.

Do the FSMs have a diagnosis chart?

Blend door failure is a very common problem among Chevy vehicles, as well as blend door actuator motor malfunction. Typically, to repair a broken blend door, a mechanic will disassemble the dash and replace the plenum box. It is a very costly and labor intensive fix. There is a DIY solution from www.heatertreater.net which provides steel hardware as well as instruction to make the repair by removing the glove box rather than the entire dash. It is worth looking into if you think you might have a broken blend door.

Thanks for the link, FB. But I'm not sure that would solve the problem. I plan to take things apart some day and see for myself.

While the blend door op. is sporadic, I also have issues with what sounds like the compressor clutch engaging when the heater is selected. Or uncommanded a/c operation. Or the rear air door popping; all the way in the back. Or vent air only. I need to determine which (if not all) parts of the system are failing (controller, door motors, temperature sensors...)

I hear ya, Marty. Sometimes the diagnostic work can be troublesome, particularly with dual control climate systems. There are many components that can fail, and the codes aren't always clear. Just keep in mind that if you are frustrated with the amount of time, work, and money that you are having to put in to solve this problem, there is another option out there that will get your heat/AC functional again. The heatertreater kit converts the dual climate control to a single control system, which is operated off of the bottom actuator motor that can be accessed by removing the glove box. This makes the repair relatively easy and is preferable to other labor intensive methodology. The reason that I believe your problem is related to blend door or blend door actuator malfunction is that these are industry wide common problems, and would be much more likely than something like a dysfunctional climate control module. Good luck!