For 1990-2009 cars

### Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring, Node Alive messages. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor for that module's Node Alive message. Each module on the class 2 serial data circuit which is powered and performing functions that require detection of a communications malfunction is required to send a Node Alive message every 2 seconds. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx, where xxx is equal to the 3 digit identification number, is set.

The control module ID number list below provides a method for determining which module is not communicating. A module with a class 2 serial data circuit 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. The modules that can communicate will set a DTC indicating the module that cannot communicate.

Control Module | ID Number
--- | ---
Body Control Module (BCM) | 064
Electronic Brake Control Module (EBCM) | 040
Instrument Panel Cluster (IPC) | 096
Radio | 128
Powertrain Control Module (PCM) | 016
Remote Control Door Lock Receiver (RCDLR) | 176
Vehicle Theft Deterrent module | 192
Sensing and Diagnostic Module (SDM) | 088

When more than one Loss of Communication DTC is set in either one module or in several modules, diagnose the DTCs in the following order:

1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
2. The DTC which is reported the most times.
3. From the lowest number DTC to the highest number DTC.

### Conditions for Running the DTC

• | The vehicle power mode, ignition switch position, requires serial data communication to occur.

• | The Module supply voltage is in the normal operating range of 9-16 volts.

### Conditions for Setting the DTC

A message from a learned identification number has not been detected for the past 5 seconds.

### Conditions for Clearing the DTC

• | A current DTC will clear when a Node Alive message from the failed identification number is detected on the class 2 serial data circuit or at the end of the current ignition cycle.

• | A history DTC will clear after 50 ignition cycles with no repeated failure.

### Diagnostic Aids

When multiple Loss of Communication DTCs are set concurrently, the likely cause is a bad connection at the Fuse Block-Left I/P class 2 serial data connector. Refer to Control Module References

### Test Description

The number below refers to the step number on the diagnostic table.

1. The module which was not communicating due to an open in the class 2 serial data circuit may have set Loss of Communication DTCs for those modules that it was monitoring.

2. The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs from these modules to avoid further misdiagnosis.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Link Connector Schematics Connector End View Reference: Master Electrical Component List and Inline Harness Connector End Views in Wiring Systems
1 | Test the battery positive voltage circuits and ignition voltage circuits of the module that is not communicating for B+ or a high resistance an open or a short to ground. Refer to Control Module References in Body Control System for the appropriate schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the problem?

Go to Step 6

Go to Step 2

2 |

1. Turn OFF the ignition.
2. Test the ground circuits of the module that is not communicating for a high resistance or an open. Refer to Control Module References in Body Control System for the appropriate schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 3

3 | Inspect for poor connections at the battery positive voltage circuits, the ignition voltage circuits, the ground circuit, and the class 2 serial data circuit of the module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4 | Test the class 2 serial data circuit of the module that is not communicating for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. For the applicable schematic, refer to Control Module References in Body Control System. Did you find and correct the condition? | Go to Step 6 | Go to Step 5
5 | Important: Perform the module setup/programming procedure if required. Replace the module that is not communicating. Refer to Control Module References in Body Control System for the appropriate repair instructions for the module replacement. Did you complete the replacement? | Go to Step 6 | --
6 |

1. Install a scan tool.
2. Turn ON the ignition with the engine OFF.
3. Select the Display DTCs function for the module that was not communicating.

Does the scan tool display any DTCs that do not begin with a "U"?

Go to Control Module References in Body Control System for the applicable Diagnostic System Check

Go to Step 7

7 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 8 | --
8 | Select the Display DTCs function for the modules which had the Loss of Communications DTC set. Does the scan tool display any DTCs which do not begin with a "U"? | Go to Control Module References in Body Control System for the applicable Diagnostic System Check | Go to Step 9
9 |

1. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.
2. Use the scan tool in order to clear the DTCs.

Did you complete the action?

System OK

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