Modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have programmed information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module. DTC U2103 will set when fewer modules are detected on the buss before the modules can learn the other module's IDs. If the communication buss is opened, or if the a modules power supply is lost when the vehicle is in the OFF power mode, and then placed in the RUN power mode, U2103 will set in specific modules.

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

The module uses a default value for the missing parameters.

Schematic Reference

Control Module References

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

Scan Tool References

Data Link Communications Description and Operation for Scan Tool information.

Important: If more than one module is not communicating use Data Communication Schematics to determine which module is closest to the DLC. Start diagnostics with that module.

1. Using the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector of the module that is not communicating.

Important: Only the high speed GMLAN modules have a wired serial data wake up circuit. Refer to the module schematics to identify which modules have a wake up circuit. If the module that is not communicating does not have a serial data wake up circuit, proceed to step 4.

3. Ignition ON, verify that a test lamp illuminates between the high speed GMLAN serial data wake up circuit OR the accessory wake up serial data circuit 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.

4. Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground.

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

5. Verify that a test lamp illuminates between all ignition voltage circuits and ground.

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

6. Test for less than 1 ohm of resistance between the module ground circuits and ground.

⇒	If greater than 1 ohm, test the ground circuit for an open.

7. Test for less than 1 ohm of resistance in the GMLAN serial data circuit(s) between the non-communicating module and the module that displays the DTC.

⇒	If greater than 1 ohm of resistance, test the GMLAN serial data circuit(s) for open/high resistance.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.