Theory Of Operation
To help improve fuel economy and emissions the Powertrain Control Module (PCM) monitors the HVAC settings for Coolant Temperature Model robustness. The PCM receives HVAC signals bussed from the HVAC Control Module. These are CAN C signals which indicate the Blower Speed and Driver Side Blend Door Position. The PCM is required to diagnose signals and set codes for any failures. On a vehicle that is equipped with dual zone HVAC Systems, the PCM only performs diagnostics on the Left/Main Blend Door Actuator.
If the HVAC Module loses and regains it's power or ground supply it will run a re-calibration procedure on the next ignition on cycle. While the re-calibration procedure is running the blend door signal voltage broadcast over the Bus will be at 0.0 volts for a few seconds. The PCM may erroneously set the Blend Door Control Circuit Low DTC during this time.