2000 gmc sierra 1500 5.3 v8. a/c heater control problem

I had this happen on my 2004 also. I have the climate zone control module that allows for adjustment for driver or passenger temps. I had to cycle between hot and cold on the passenger side and switch it between defrost and feet, then power off everything for a long period of time using the power button on the control, this finally got the solenoid to become unstuck. If that doesn't work then you have to get into the heater/air ducts under passenger side, which is difficult, to manually unstick or replace the locked solenoid.

Usually a sign of a faulty blend door.
The blend doors direct air thru the heater core and out thru the vents.

Suspect temperature control servo If stripping noise is heard when changing from hot to cold and back,replace servo.

sounds like you have a bad actuator motor that is what moves the door in heater box to change from hot to cold

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

Check in the one heater hose will be a control valve. Check the operation of the valve(this is the heater control valve). If the lever on it appears to be moving. Take the valve out and manually check that the valve is opening and closing the whole way. If not replace the valve and recheck. If it is the heater core may be clogged not allowing the hot coolant to flow thru so you can get heat.

check dial switch, check valve that lets water run through heater core

check your coolant level on the radiator

I experienced the same head ache. First attempt was to replace the thrmostat and after taking it apart I realized that there was nothing wrong with it. I then removed the plastic cover/panel located under the dash board on the passenger's side. (only a few 4-5 smalls screws) Removal will enable you to see the on/off switch/gate moving/rotating as you turn the knob to hot or cold positions, on the control panel. If that switch/gate seems to work ok, then the trouble is within the heater core. Many mechanincs suggested that I replace the heater core. A very expensive proposition.
I checked the heater core supply/intake hose and found it hot to the touch, while the return hose was nearly cold. It was definitely a heater core blockage. I removed both ends (supply and return) hose connections/clamps. They are located near the fire wall in the engine compartment. A special tool is very helpful to remove those clamps easily and to replace them as well. It can be done the old fashion way but this tool was a life saver and reduced work time to just minutes. (Tool is $70.00 bucks at auto parts store,so I borrowed mine from a mechanic/friend)
I then hooked up a garden hose to the return end of the heater core and reversed the flow of coolant/water through the heater core. My truck is a Sierra 2000 and I refuse to see how a heater core could get clogged up with clear coolant alone. To my surprise a flat piece of rubber about the size of a quarter blew out of it and the flow of water became unobstructed. I reconected the ends of the hoses to the heater core, replaced the lost antifreeze & everything returned back to normal. No money spent for something most mechanics suggested I replace control valves/switches ($280.00) replace heater core ($500.00) etc etc. I am glad I was curageous enough to tackle this myself and save a few pennies. Good luck, and I hope this will work for you.

The clutch in compressor is controlled by a pressure switch. Start there.