Code B440, HVAC - Air Mix Door Circuit Problem: Notes

Code B440 is set by the BCM if it is unable to move the air mix door. The BCM requests the door to move over the data circuit No. 800 to the programmer. With the ignition on, the programmer supplies voltage (5 volts from BCM circuit No. 705) to the motor that drives the air mix door. The BCM monitors a feedback potentiometer on the DC motor which varies between 0 and 5 volts depending on its position. If the door is requested to move, but the feedback voltage does not change, the BCM stores Code B440 to indicate that the door is not responding to its command. If the air mix door feedback indicates it is near the hot or cold extreme, the code will not set if it is commanded to move to that extreme.

Operation of the air mix door can be evaluated in BCM data display by observing actual air mix door position (BD23 "ACT MIX DR") while changing the program number (override BS01 "PROGRAM NO").

1. BCM data value BD23 displays actual air mix door position. The normal range is 1-99%. The BCM override value BS01 is for the program number. As the program number is changed from 0 (maximum A/C) to 99 (maximum heat), the door position should vary between its mechanical limits of travel.
2. Checks to see if the fault could be due to the 5-volt reference and ground circuit or the sensor circuit.
3. Checks to see if the fault is due to the programmer or the sensor circuit.
4. Checks to see if the fault is due to the circuit or the BCM.