Vito w639 heater temperature control problem

[peterp50]

Newbie here, perhaps someone can help with this. My Vito W639 116 only introduces warm air into air flow when the temperature potentiometer is within the uppermost portion of the dial from about 24 to max. With the vent cover removed I can see that the motor only acts on the arm, that is no doubt attached to a flap, when the control is near the top of the range. so comfortable temperature is difficult to achieve. Sometimes it acts from about half-way, and sometimes there is what I would describe as a "hunting" motion where the motor moves back and forward a little while the temperature control remains in one position. I am happy that the coolant temperature is correct, so I suspect the potentiometer, the motor or a sensor. Thank you for any advice.