Diagnostic Instructions

•	Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

•	Review Strategy Based Diagnosis for an overview of the diagnostic approach.

•	Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

Refer to Control Module U Code List.

Circuit/System Description

The serial data is transmitted over a Local Interconnect Network (LIN) single wire network circuit bus between a master control module and other control modules within a particular subsystem. If serial data communication is lost between any of the control modules on the LIN bus network, a no communication code against the non-communicating control module will be set. A master control module is the one that reports the non communication code. Note: a loss of serial data communications DTC does not represent a failure of the master control module that set it.

Diagnostic Aids

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A control module may have a U-code stored in history that does not require any repairs. Problems with late or corrupt messages between control modules may only be intermittent without any obvious symptom or problem. This does not mean the control module is faulty. Do not replace a control module based only on a history U-code.

•	Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.

•	Communication will be available between the master control module and the scan tool if there is a loss of communications with any of the other control modules on the LIN bus network.

•	An open in the LIN bus serial data circuit between the splice pack and a module will only affect that specific module. This type of failure will set a loss of communication DTC for each control module affected and the other control modules will still communicate.

Reference Information

Schematic Reference

Data Communication Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

•	Circuit Testing

•	Connector Repairs

•	Testing for Intermittent Conditions and Poor Connections

•	Wiring Repairs

Circuit/System Testing

Note
•	Use the schematic to identify the following:

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Control modules and LIN devices the vehicle is equipped with

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Control module and LIN device locations on the serial data circuit

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The control module serial data circuit terminals and LIN device B+, ignition, ground, and serial data circuit terminals

• Use the Control Module U-Code list to determine which control module is not communicating with the master control module. Refer to Control Module U Code List.

• A loss of serial data communications DTC does not represent a failure of the master control module that set it.

1.	Ignition OFF, disconnect the harness connector at a LIN device that is not communicating.

2.	Test for less than 5 Ω between each ground circuit terminal and ground.

3.	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.

5.	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 LIN device for a short to ground. If the circuits test normal, replace the LIN device.

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

7.	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 LIN device for a short to ground. If the circuits test normal, replace the LIN device.

8.	Ignition ON, test for 2-11.5 V between the LIN serial data circuit and ground.

9.	If greater than the specified range, test the LIN serial data circuit for a short to voltage. If the circuit tests normal, replace the master control module.

10.	If less than the specified range, test the LIN serial data circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the master control module.

11.	If all circuits test normal, replace the LIN device.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

•	GMLAN Wiring Repairs

•	Control Module References for control module replacement, setup, and programming