2000 GMC Sierra 5.3L. Heater is stuck on full heat. It is not responding when I turn the dial to make colder.

Hi Dennis, I'm glad to help!

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.

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

thermostat stuck or heater control valve bad..you may have air in the system too

Please see my tip at http://www.fixya.com/cars/r6235876-no_heat_fan_blowing_cold_air . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.

You HVAC unit is not getting the heat from your engine coolant. A valve is stuck or there is blockage in the heat exchanger core. I'm assuming you fan and vent divertors/Controls are working correctly. Does AC work?

two things--first replace your cabin air filter.second reverse flush your heater core with a garden hose and that should remove the build up of material in your heater core and restore the flow,hope this helps

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

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.

After replacing the thermostat I pulled the heater hoses off and blew air through the heater core at firewall. Can only blow in left side as directional valves will prevent opposite. Chunks of stuff came out with coolant. I then put a piece of hose on the left side and shot water through until it came out clear.
Put it all back together and heat works fine.

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