I have a 00" yukon with no a/c in the front but the back a/c works the front just blows out hot air?

This year Yukon uses an electric heater door accuator. It sounds like the heat "blend" door accuator is not working. It is easy to replace. Right above your gas pedal, on the l/h side of the heater box, yoy will find the motor. It is 3" square, black with 5 very small wires coming from it. Unplug the wires, undo the three screws and pull the motor away from the htr. box.