Truck Heat

Bhesch · August 27, 2019

I have a 2011 GMC Sierra that does not want to blow heat. I have did the majority of troubleshooting and have a general assessment. First check was the antifreeze. It was low so I topped it off. Then I thought it was the thermostat, but my gauge shows 210 and both lines to the heater core are hot. Next I checked the blend door actuator under the passenger side and found it not moving. I ordered a replacement but it also did not move. So I found the AC Delco video on testing them and both my current and the new one tested out working. I remove the battery connection and let everything reset. It started working then, but I noticed my outside temp gauge in the truck said 55 degrees. But every function worked fine. AC, defrost, and heat, both dash and floor. Then later when the outside temp reached 76, the heat was gone. I did more testing and found that the input connector is getting power and checked plugs in the back of the control unit. So now yesterday and today while driving to work it was around 64 and 67 in the morning. I tried the temp controls heat, up and down, defrost, and all are working. So my question is, would there be a sensor that is preventing my truck heat from working when the outside temp is a certain temperature? In the winter I can set the heat to 74 and sweat myself out, but now, I need to turn it to 80 maybe I found this morning. I am stumped and am about to take it to the dealer for diagnostics. I thought about just changing my thermostat and flushing coolant since I am just at about 100K miles (97K actually). I do not get any check engine lights either. Any direction or info would be great.