01 gmc sierra heater is stuck on - 2001 GMC Sierra 2500HD

It sounds like the bypass valve is stuck open. Check bypass valve and vacuum hoses attached to it.
Hope this helps.
Bob

Check the fuse but it is possible that a bulb can be out

Usually a sign the blend door is stuck in the heater box. The blend doors direct air thru the box and the vents.

sounds like your hot water by-pass valve is stuck open

If you are talking about the exhaust manifold to pipe flange gasket. Yes they are the same.

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

Defrost is the default position. Is the control a autyomatic temperture control unit? ( you set the temp and select automatic) if it is the system is completely electronic. You push the buttons and the control still says defrost or can you select something else and the control shows it but it just keeps coming out the defrost? If the control changes and the air comes out the defrost anyway and the cheack engine light isn't on, then the problem can be blown fuses or an air door actoater not working. Does the dash emit a clicking sound? if it does with the covers off the bottom of the dash reach around touching the different pieces till you touch the idem that is clicking. this is an acutor motor, or blend door motor. they have gears in them that wear out and break the motors aren't repairable and you just replace the ofending motor. the door that is moves can be stuck. and may have to be freed up with blaster, WD40, or other rust buster till it moves easy. maybe after getting the door to move smoothly you can get the old motor to move the door wiithout having to install a new one.

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.

THERMOSTAT REPLACE IF NOT SEE IF HOSES ARE CLOGGED SHOT OF AIR COULD HELP OR DISSCONNECT BATTERY CABLE 10-15 MIN CONNECT BACK TRY HEATER

the small radiator just inside the cab, where the hoses go to the heater is leaking. You may try using stop leak (many don't recommend this) or take off the heater radiator. it will probably get worse when you turn on the heater and fog up the windshield if not fixed. hope this helps.