There are seven sensors that affect the digital HVAC control system.
- The interior temp sensor with a fan with a cover that looks like a microphone or tiny speaker grille in the headliner above the drivers' head. GM calls it an Aspirator. ACDelco GM PN is 22834330
- The Ambient outdoor temp sensor in the framework under the RH turn signal lamp. ACDelco 25775833
- Four Instrument Panel Air Duct Air Temperature Sensors.
1 Upper Left, 2 Lower Left, 3 Upper Right, and 4 Lower Right.
- Sun load sensor in the centre defrost grille in the dash. Usually affects the AC.
Look at the outputs from these seven sensors with your scan tool.
My gut tells me the tiny aspirator fan has failed as that is a common issue.
That sensor assembly is not super cheap. At $30-$40 I would check it first. I wouldn't just load it in the parts cannon and say your Hail Mary.
The other thing that could affect your Heat is an air bubble in the heater core. It's common for my W-Body Impala but I haven't had that issue with my truck.
Park the truck on a fair incline and run the engine at 2,500 RPM while filling the coolant tank to the MAX mark with Dexcool.
Park it on the level and draw off any excess til the coolant is just under the max line at operating temp.
Your other option is to use a vacuum cooling system filler tool.