Example 1:

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From the DTCs stored in the memories, you have determined that control module 1 is not communicating with control modules 2 and 3.

Fig 1: Schematic - Control Module 1 Not Communicating With Control Modules 2 & 3

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Fig 2: DTC Memory - Control Module 1 Not Communicating With Control Modules 2 & 3

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Switch ignition off.
Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
See appropriate System Wiring Diagram article.
Replace control module 1 if no malfunctions can be found in the bus wires.