The serial data circuit is used to communicate information between each module connected to the serial data line. Each module is assigned a specific recognition code (source ID). This code is used to identify which module is communicating on the serial data line. Modules communicate with others by periodically sending messages on the serial data line. These messages contain specific information (or commands) which allow other systems to function properly. When a module receives a message it also learns the source ID of the system that sent it. This source ID is used to determine which module actually sent the message and for State of Health (SOH) monitoring. SOH monitoring determines if a module is able to communicate properly. Modules periodically send a SOH message to each other containing their source ID. If a system fails to receive a SOH message from a module that it communicates with, a DTC identifying which module that did not respond will be set. Any module that also cannot communicate properly, will also store the appropriate communication DTC that was assigned to that module. The HVAC Control Assembly also monitors the serial data line for an open or short (to ground or voltage) condition. If the HVAC Control Assembly detects any of these malfunctions as current, the HVAC Control Assembly will store a DTC.

The numbers below refer to the step numbers on the diagnostic table:

2. This test checks if communications can be established between the scan tool and the other systems connected to the same serial data line.

3. This test checks if communications can be established between the scan tool and the IPC.

4. This test will check for any opens in the serial data line between the BCM and the PCM or IPC. It also checks for any intermittent malfunctions associated with the serial data line.