There is the position motor that controls where the air comes out and the blend motor. Since it is only working when at max position, that means the potentiometer is worn out or damaged in the blend motor. Good news is they are dirt cheap, bad news they are a PITA to replace.
When switched to max, the motor is fully closing on the heat exchanger, but when changing to a warmer temp, it sounds like the motor doesn't know where to stop and is fully opening the door. You can try doing a reset by disconnecting the battery for a while. You can also hook a scan tool up and look for codes, a bad motor will set a code.
When switched to max, the motor is fully closing on the heat exchanger, but when changing to a warmer temp, it sounds like the motor doesn't know where to stop and is fully opening the door. You can try doing a reset by disconnecting the battery for a while. You can also hook a scan tool up and look for codes, a bad motor will set a code.