The serial data circuit is the means by which the control modules in the vehicle communicate with each other. Once the scan tool is connected to the serial data circuit through the DLC or CAN connector, the scan tool can be used to monitor each module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). When the ignition switch is in RUN, each control module communicating on the serial data circuit sends a state of health (SOH) message to ensure that the control module is operating properly. When a control module stops communicating on the serial data circuit, for example if the module loses power or ground, the SOH message it normally sends on the serial data circuit disappears. Other control modules on the serial data circuit, which expect to receive that SOH message, detect its absence; those control modules in turn sets a DTC associated with the loss of SOH of the non-communicating control module. The DTC is unique to the control module which is not communicating therefore a loss of serial data communications DTC does not represent a failure of the module that set it.

The system voltage is between 9-16 volts.

A supervised periodic message that includes the transmitter module availability has not been received.

Specific subsystems will not function.

Schematic Reference

Connector End View Reference

Data Communication Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Verify that following BCM DTCs are not present:

Note: Use the schematic to identify the following:

1. Determine the module that is not communicating.
2. Ignition OFF, disconnect the necessary harness connector of the module that is not communicating.
3. Test for less than 5 Ω between each ground circuit terminal and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

4. If equipped, verify that a test lamp illuminates between each B+ circuit terminal and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the control module.

5. If equipped, ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground.

⇒	If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the control module.

Note: Only the high speed GMLAN modules have a serial data communication enable circuit OR an accessory wakeup serial data circuit, and the BCM is the output for these circuits. Refer to the module schematics to identify which modules have these circuits.

6. If equipped, ignition ON, verify that a test lamp illuminates between the communication enable circuit terminal OR the accessory wakeup circuit terminal and ground.

⇒	If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuits test normal, replace the BCM.

7. Ignition OFF for 60 seconds, test for less than 5 Ω between the serial data circuit terminals and the DLC terminal listed below:

•	Class 2 serial data circuit terminal 2

•	Low speed GMLAN serial data terminal 1

•	High speed GMLAN serial data terminal 6 or 14

•	J1939 serial data CAN connector terminal C or D

⇒	If greater than the specified range, test the serial data circuit for an open/high resistance between the non communicating control module and a module setting the DTC or a serial data splice pack.

8. If all circuits test normal, replace the control module that is not communicating.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.