2006 GMT800 (Chevrolet Silverado 2500HD 6.6) - Climate Control Diagnosis

Digital dual zone climate control system common to period Silverado, Sierra, Suburban, Denali, Yukon, Tahoe, Avalanche, etc. Repair attempt by shadetree mechanic. As far as I know this period had two systems - one was dual zone manual (two temperature sliders) and this system, dual zone auto with the digital display and two temperature dials.

Symptom - system delivers full cold to both driver and passenger zones unless zone setting is 90 deg. At 90 deg, both zones swing full hot. Swap to a junkyard CC module by prior owner (my father) did not change the issue.

I found a wiring diagram for the system online. Leaning towards a faulty "aspirator" module - a temperature sensor in the headliner above the driver. There are also four temperature sensors in the ductwork, according to the diagram. The diagram represents these all as thermistors. As a shadetree home mechanic, is there an "easy button" source for resistances of these sensors? Is there some calibration the system needs to do in order to learn all these temp sensors?

Outside/ambient temp on rearview display is correct, so that sensor is "good". Dash gauge and scangauge indicate engine temp is where it should be.

Background, my father had the truck for three years during which it always had this issue. He's very mechanically inclined and did lots of work on the truck but this issue never was a high priority to him. He did replace the CC control module as the buttons/display on the prior unit were trashed, but this did not solve the issue. Eventually he caved and took the truck to a trusted shop (not parts changers) that supposedly tested all actuators and sensors. They deemed all the hardware good but said the system needed a "calibration" and suggested the dealer would have the right computer to do so. Dealer willingly charged their diagnostic fee, spent a week with the truck, and insisted on new engine thermostats. Quoted $900 for the job with no guarantee this would fix the CC issue. The truck did occasionally throw a "did not warm up in specified time" code in the winter if unloaded, so they weren't entirely unfounded in their finding. Apparently, they saw the code and set out to address it even if not related to the customer complaint. Thermostats were unknown age, so owner replaced thermostats. No change in performance. I now own truck and am an advocate for warm wife = happy wife.

I did find on one truck one time the temperature sensor you describe in the headlinder, but I don't remember exactly what the problem with that truck was. There was most likely a code that lead me to test there.
If you have a scan tool that can display HVAC data, a -40°f from that sensor is a dead give away to an open thermistor circuit.
Otherwise, disconnected with the ignition on, there should be a 5v feed and a ground. Connected, the 5v feed will drop depending on the temperature. In a thermistor circuit, the feed is the signal circuit. I don't have a good spec for you, but if there's more than one temp sensor, they should all be just about the same temperature in the cab of the truck, and therefore should have very close to the same voltage while connected with the ignition (and heater control) on.
Without some trouble codes and live data, this could be a very difficult issue to nail down. There is probably a sequence of button presses on the HVAC control head to put it in a self check mode where it may flash some codes at you if you Google hard enough

Without having a scanner, I can probe with DMM to get resistance to ground, which I was hoping would lead me down the right path.

My aspirator reads ~2k at/near room temperature and goes up when warmed (as one would expect.) I just don't know if that's good, bad, or neither (which I would suspect if the CC unit gets calibrated - IE, you tell it "hey, vehicle has been sitting in a 60 deg shop. Whatever all those sensors currently read, take that as 60 deg.")

Noah, if I understand you correctly, the CC unit puts either regulated current or voltage to one side of the thermistors with the other side grounded and measures the result to determine temperature.

My logic is, if the aspirator failed (high resistance) then the CC thinks the cabin is hotter than reality. As such, it always aims towards full cold no matter the temperature set...except when set to max (90 F) in which case it then flops the blend doors to deliver full heat. There would be no middle ground.

Well,

Parts cannon. I grabbed a new aspirator. It appears the aspirator sensor is a negative coefficient thermistor. The new one is about 12k ohms at ambient outside right now (40F) and about 5k if I breathe on it a few times. My old one is much lower resistance. I'll report back results once I can drive the truck, get it to temperature, and observe behavior.

That said, I sure wish it was easy to get my hands on a resistance spec for the sensors. It would make diagnosing this system child's play.