[Cliff8928]

Originally Posted by GM

DTC U1001-U1254 Loss of XXX Communications

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 10 seconds, a DTC U1xxx (where xxx is equal to the three 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

Electronic Brake Control Module (EBCM)

040

Body Control Module (BCM)

064

Sensing and Diagnostic Module (SDM)

088

Instrument Panel Cluster (IPC)

096

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:

Current DTCs before history DTCs unless told otherwise in the diagnostic table.
The DTC which is reported the most times.
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.

Conditions for Setting the DTC

A message from a learned identification number has not been detected for the past ten 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 upon receipt of a scan tool Clear DTCs command.
Diagnostic Aids

When multiple Loss of Communication DTCs are set concurrently, the cause is likely to be two opens in the class 2 serial data circuit. Use the Control Module ID Number list in order to determine which modules are not communicating. Use the class 2 serial data circuit schematic in order to determine the location of the opens. Refer to Data Link Connector (DLC) Schematics .

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