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 Is Not Communicating With Control Modules 2 & 3

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Fig 2: Example 1 DTCs Stored In Memory - Control Module 1 Is Not Communicating With Control Modules 2 & 3

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Switch ignition off.
Disconnect the control modules connected across the bus lines and check the bus lines for an open circuit.
See appropriate System Wiring Diagram article.
If you cannot find a malfunction in the bus lines, replace control module 1.