Code B440, Air Mix Valve (Door) Circuit Problem: Notes

Code B440 is set by BCM if commanded air mix door position is between 30 percent and 80 percent, but actual air mix door position is not within 2 percent of commanded position for 60 seconds. The BCM commands the programmer to move air mix valve (door) over data circuit No. 800. With ignition on, BCM supplies 5 volts on circuit No. 705 and ground on circuit No. 736. A motor in programmer drives air mix valve (door). The BCM monitors door position through a feedback pot on DC motor which varies between zero and 5 volts depending on air mix door position. If air mix valve (door) feedback indicates it is near hot or cold extreme, code will not set since mix door may be at its limit of travel and not able to reach commanded door position.

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

1. BD23 display indicates actual air mix valve (door) position. The normal range is 1-99 percent. BCM override value BS01 display is for program number.
2. Checks to see if fault could be due to 5-volt reference and ground circuit, or sensor circuit.
3. Checks to see if fault is due to programmer or sensor circuit.
4. Checks to see if fault is due to circuit or BCM.
5. This step checks to see if open circuit reading is due to circuit No. 736 or circuit No. 705.