Did you ever find a solution to this problem? My 2001 Sierra does the same thing; if I turn on the heat to full hot it remains stuck for a while. Sometimes it sticks for a couple of minutes, other times it sticks for a couple of days. As long as I don't turn it fully to hot though, it doesn't stick. It has gotten worse recently.
If you dont hear a switch over when you turn dial the system probly needs a charge possibly low pressure.if its just the dial knob need to open the panel and see if its making contact while turning knob see if it switches.
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Dennis, there are only 4 possibilities when you have NO Heat. (1) Your are low on coolant. Check your Radiator and see if it is full. (2) The heater core is clogged. First run the engine until it reaches operating temperature with your heater turned on, then grab one heater hose and compare it to the other. If it's clogged one will be extremely hot and the other somewhat warm. If it's not clogged both will be extremely hot. (3) You could have a blend door problem. Which means the door could be stuck in the colder side. It also could be broken or the motor is bad. (4) The control head could be bad and not actuating the door. If you like call me 314-374-5218 I'm in St Peters. Hope this helps and have an awesome day Dennis.
If your Sierra has a Duramax, it may be the intake air heater or the fuel heater cycling on and off in the cold weather. But if its pulling the voltage down to 12V after warmup, I would suspect a poor battery. As suggested, get a load test at autozone or ??
make sure antifreeze is full. run engine 10 min. verify engine is around 195 degrees. if not, try replacing thermostat. if it is running at 195, then maybe heater core problem or heater control not functioning properly