My 2000 gmc sierra truck heater heats up but sometimes don't turn down

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

the valve that allows the hot water into the heater core is not opening. most likely electronic issue. also check all vacuum and electric lines.

Try to disconect the battery cable(negative) for about 15 seconds to reset the ac/heater control.

you have a couple of problems first of all your drivers sd=ide heated heat pad is probably faulty, for your heating problem you may have to get the hvac doors recalibrated.

your heat/cool and vent selection options are controlled by engine vacuum through small clear tubes attached to the rear of your climate control interface. Other vehicles have small steel pull wires that operate the desired changes. You might try removing the trim and any fasteners affiliated with the interface/console. GENTLY pull outward and look to see if any of these tubes are pulled off or cut/damaged. Did anyone recently install a new stereo in the vehicle? If so, the vacuum tubing and or pull wires that change your outlets could have been damaged when the stereo unit was pushed into place. Hope this helps

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

Have you tried flushing out the heater core with a garden hose?

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.

check your coolant level on the radiator

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