Been watching your videos forever. Really enjoyed your perspective and take on the automotive industry. I used to work at Denso and I feel that you know more about cars that most automotive engineers. lol Of course we might know one part of the car better. 

I have a 2013 Tahoe PPV. I notice a coolant leak. After pressure testing it I notice that the radiator was bad so I put in a cold case radiator. A couple of weeks after installing it. I noticed that the warning light at the bottom was saying Engine Hot AC Turned Off. What was odd was the engine temp gage on the dash said that the engine had no heat at all. the gauge was pinned at the lowest level. I consulted cold case and they mentioned to restrict with a hose clamp the smaller hose on the passanger side. I did this and it did not resolve the issue. They suggested the next step was to replace the thermostat. I had AAA mechanics check it and they suggested the same thing(They claimed that the coolant sensor resistance was in spec was ok based on their check). But since they wanted 700USD to swap a thermostat I did the swap myself in the parking lot.

Fast forward like 5 days Im greeted with the Engine Hot AC Turned Off. This time I notice the Resivior was like way over the level I filled it to. One other issue I have is the larger Radiator hose was not on very tightly. I ordered one prior to the issue I will install it soon but I have a feeling that this is not going to fix the issue.

The code I have is P0128
engine coolant temperature is below the thermostat regulating temperature