I have a 2006 GMC Sierra 1500, v6, auto trans, 2wd, manual climate control, extended cab, 6.5 bed.
I have replaced correctly with the correct parts, the mode door actuator (drivers side, under and between the steering column and center dash and yes the fin is still easy to move and not locked or strained when trying to move it without the actuator in) the temperature actuator (passengers side just above the middle seats floor) and the Dual side actuator? ( Passengers side inside the dash behind the left air vent), also I have replaced the blower motor resistor.
When messing around with the temperature actuator I am able to still get freezing and blazing hot temperatures, and the volume of air coming out of the vents is normal as well.
My problem is the mode (face vents to floor to defrost) will switch to defrost vents on its own after a few start ups. The AC still works through the defrost vents but very softly and the heat works as well.
I have taken out the AC fuse and did the ritual ( turn the ignition over but not on and wait a minute and all that) for that from the in cab fuse box, I have done the hvac/ e something fuse removal and ritual ( I have tried 3-4 different protocols from forums for both fuse and battery removals) for that from the under hood main fuse box, and I have disconnected the battery. I have tried them all together and sperate and different orders and some combos make the controls go back to normal for a time before they eventually go back to blowing out the defrost vents.
I have tried everything from forums and the dealer diagnosis was the actuators but I had new ones in then and since then replaced them all again. I was hoping it was going to be that recall issue that burns out some part over time or at high enough temps but the deals ruled that out.
Assuming you were given the correct mode actuator (there are two options and are dependent on whether your truck has a factory floor mounted center console with rear vents or not), the chances are that there is a poor connection somewhere. Keep in mind that anytime the system sees an actuator position sensor outside of the commanded range and it cannot compensate for it it will drive the actuator to the defrost position due to safety concerns. The most likely locations are at the junction block connectors at the passenger dash end cap. Disconnecting the connectors and reinstalling several times can sometimes correct for a terminal fretting condition (light corrosion that is not visible). Also inspect for visible corrosion or loose terminal fit. There can also be a poor connection at the actuator harness connector. You can check/address this in two ways. Gently wiggle the harness connector within one minute of turning the ignition on to see if the actuator will move in response to the harness movement. Second ,you can take the time to remove each terminal one at a times and tighten the tension on each terminal.