This post might be useful
Have a read of this, not saying its the problem on your car but you never know.
I think I've solved this problem in my '00 M Coupe. It's not caused by the HVAC control unit (the unit with the temp knob) or the bowden cable that connects it to the blend door arm on the heat box. The problem appears to be caused by the blend door catching on something in the heat box when the door is at, or possibly slightly past, the full cold position.
Fortunately the cable is adjustable at the blend door arm connector. To get at it you have to remove the kick panel (which includes the glove box) and a metal bracket that blocks access to the cable connector. The bracket is held on with three screws (10mm socket and extension needed):
Once you have those pieces removed, move the temp knob to the full hot position so the connector and control arm are in view:
The connector uses spring pressure to grip the cable. Adjust the cable by bracing the connector with a screwdriver while you grip the cable with needle nose pliers and slide it to the right in the connector. Use the screwdriver to avoid putting pressure on the plastic control arm while you move the cable. I moved the cable approx. 1/8", which was just enough to prevent the blend door inside the heat box from sticking. It's a good idea to adjust the cable, move the temp knob to full cold, and then let it sit for 10 minutes or so before testing it by turning up the heat knob. In my car the blend door sticks more if it's had some time resting in the cold position.
A couple additional tips for adjusting the cable:
- After changing the position of the cable in the blend door connector, turn the temp knob back and forth between full cold and full hot to make sure the blend door isn't hitting or binding against anything inside the heat box.
- You may want to put something on the cable to prevent it from sliding in the connector. In the third picture you can see a small gap between the right edge of the connector and the cable's bent tip. I found a small wire tie that just fit in that gap, preventing the cable from moving again.
