blend door actuator. about the size of a pack of cigarettes, about $40 on rockauto/com, and a pain in the *** to get to.
It's a small solenoid / gear train, which controls the flap door that mixes heater core and cabin air to the temp you call for. The geartrain gets overdriven, or a gear breaks, and the door no longer moves in accordance with the learned range and the resulting temps are nothing like what is set. The 'reset' simply wipes out the learned range of movement. When you reconnect, restart, the controls move from limit to limit and that's what the controls use as a new baseline for movement. If your actuator geartrain isn't busted, this should work. If it doesn't, you'll need to replace it.
Up to '02, the blend actuator is deep in the dash, practically inaccessible. It's put on before the dash goes in. You'll have to pull out much of the dash to get to it. Not difficult, just a pain.
2003-on with the zone temp controls, there's a blend door (& actuator) for each front seat. There is also a Mode actuator which controls Defroster / Vent / Heater Feet. It's high above and inboard of the gas pedal, can be got to with a little duct removal. Mine busted, some details in my 'Hey Vortec Guys' topic. My repair didn't work for long, I have a new part coming in the mail.
info starts on pg2
http://www.expeditionportal.com/for...-Guys!-Sierra-pickup-Suburban-Yukon-etc/page2
'05 Tahoe, the driver side blend is hard against the transmission hump, passenger front corner of the center console. Has a 7mm (8mm?) bolt thru the middle of it and takes small hands and a mini ratchet to get it out.