GMC Repair: 2001 Yukon heater control, gmc yukon, accelerator pedal

Question
2001 GMC Yukon FWD 5.3L.  The Heater control will not redirect air flow to floor or defroster, airflow only from a/c vents.  When switched in and out of defrost mode, the air intake does does switch between outside air and recirculate modes but no change in direction.  A/C works, as does heat control and fan speed.
Thanks

Answer
Hi
sounds like the Mode Door Actuator is your POSSIBLE fault.
When they fail they do this same symptoms.
It is located on the driver side above the accelerator pedal (gas pedal) towards the center.
It is a common issue.
I Hope this helps
Good luck
Pawl