Just because the electronic heater contol "head" appears to be functioning does NOT mean the servomotor attached to the heat control (blend) door is following what the control "head" tells it to do. There is NO feed back from the servomotor to the control head. The control "head" simply issues a positioning command and ASSUMES the motor has obeyed. The bar graph on the control "head" display will move, even though the servomotor DOESN'T move. You may have a control "head" problem, where the "head" shows the bargraph moving and is defective, and not sending the signal to the servomotor. You may have a wiring problem between the contol "head" and the servomotor. You may have a defective servomotor, OR a mechanical problem, where the servo is moving, and the "blend door" is not. Also, the blend door is plastic, and has a rubber flange around it. Sometimes, the rubber lets go of the plastic door, so the door no longer seals, allowing cold air to bypass the heater core. LOTS of stuff to check, all located in a MISERABLE place to work!
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