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. 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 can not communicate.

Control Module | ID Number
--- | ---
Powertrain Control Module (PCM) | 016
Transfer Case Shift Control Module (TCSCM) | 026
Electronic Brake Control Module (EBCM) | 040
Body Control Module (BCM) | 064
Inflatable Restraint Sensing and Diagnostic Module (SDM) | 088
Instrument Panel Cluster (IPC) | 096
Radio | 128
Digital Radio Receiver (DRR) | 137
Communication Interface Module (OnStar®) | 151

When more than one Loss of Communication DTC is set in either 1 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 following diagnostic trouble codes do not have a current status:

- | U1300

- | U1301

- | U1305

• | The vehicle power mode requires serial data communication to occur.

### Conditions for Setting the DTC

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

### Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

### Diagnostic Aids

• | An intermittent open between the inoperative module and a star connector may cause this code to set.

• | A poor connection at the inoperative module or a star connector may cause this code to set.

• | An intermittent open in a star connector may cause this code to set.

• | An improperly powered module may cause this code to set.

### Test Description

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

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

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Important: Use the control module ID number list in order to determine which module is not communicating. Test the following circuits of the module that is not communicating for an open or a short to ground:

• | The battery positive voltage input circuits

• | The battery positive voltage output circuits

• | The ignition voltage input circuits

• | The ignition voltage output circuits

• | The switched battery positive voltage circuits

Refer to the following:

• | Control Module References for the applicable schematic

• | Circuit Testing

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 7

Go to Step 3

3 |

1. Turn OFF the ignition.
2. Test the ground circuits of the module that is not communicating for an open.

Refer to the following:

• | Control Module References for the applicable schematic

• | Circuit Testing

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 7

Go to Step 4

4 | Test the class 2 serial data circuits of the module that is not communicating for an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 7 | Go to Step 5
5 | Inspect the harness connectors of the module that is not communicating for poor connections and terminal tension at the following circuits:

• | The battery positive voltage input circuits

• | The battery positive voltage output circuits

• | The ignition voltage input circuits

• | The ignition voltage output circuits

• | The switched battery positive voltage supply circuits

• | The ground circuits

• | The class 2 serial data circuits

Refer to the following:

• | Control Module References for the applicable schematic

• | Testing for Intermittent Conditions and Poor Connections

• | Connector Repairs

Did you find and correct the condition?

Go to Step 7

Go to Step 6

6 | Replace the module which is not communicating. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 7 | --
7 |

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

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

Go to Diagnostic Trouble Code (DTC) List - Vehicle

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 Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 9
9 |

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

Did you complete the action?

System OK

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