2002 Freightliner Century heater problems

I have a 2002 Century and has the same problem. I had it fixed once before. I am going to try and figure out myself before Fall months. In the meantime, I just close the valves in the coolant lines to the cab and bunk heater cores. I'm pretty sure there is some sort of cab temperature sensor that's between the HVAC temp control knob and the water control valve. There isn't any written material trouble shooting in the manuals. So I have to go to the electronic schematics. I tried that the last time and gave up quickly because I didn't have time, and just gave it to a mechanic to fix as he already was fixing something else on the truck. BTW, I'm going to try using Gemini AI on my new Pixel phone to help. Gemini can view pictures taken from the camera and help you through projects. I didn't use the camera, but Gemini already helped me with a repair-somthing I imagine that would have taken quite a bit of time going through a Google search.

 

When it's around 45°-60° F, I leave the bunk heater valve open and turn it on as needed from a switch on the dash. Even if the cab valve is open, but the fan is off, it can get a little warm for me.