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Modules connected to the high speed 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 prerecorded information about what messages are needed to be exchanged on the serial data circuits. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module. A module with a high speed GMLAN serial data circuits malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. If any of the high speed GMLAN circuits are shorted to battery voltage, ground or between them, no communication is possible on high speed GMLAN serial data bus, and a DTC U0073 is set.
This diagnostic procedure supports the following DTCs:
A supervised periodic message that includes the transmitter module availability has not been received.
The MIL/DTC clearing depends on DTC type. For information about DTC types, refer to Diagnostic Trouble Code (DTC) Types for the 6.6L (LLY) engine.
The numbers below refer to the step numbers on the diagnostic table:
The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
Schematic Reference: Data Link Connector Schematics
Connector End View Reference: Master Electrical Component List
Did you perform the Diagnostic System Check - Vehicle?
Go to Step 2
Go to Diagnostic System Check - Vehicle
Important: Use the DTC descriptor list in order to determine which module is not communicating on high speed GMLAN serial data bus.
Refer to the following:
Did you find and correct the condition?
Go to Step 8
Go to Step 3
Go to Step 4
Test the high speed GMLAN serial data circuits of the module that is not communicating on high speed GMLAN serial data bus for an open. Refer to Circuit Testing and Wiring Repairs .
Go to Step 5
Inspect the harness connectors of the module that is not communicating on high speed GMLAN serial data bus for poor connections and terminal tension at the following circuits:
Go to Step 6
Replace the module that is not communicating on high speed GMLAN serial data bus. Refer to Control Module References for replacement, setup, and programming.
Did you complete the replacement?
Go to Step 7
Does the scan tool display any DTCs which do not begin with a "U"?
Go to Diagnostic Trouble Code (DTC) List - Vehicle
Select the Display DTCs function for the modules which had the Loss of Communications DTC set.
Go to Step 9
Did you complete the action?