Data Link 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. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an identification number with at least one critical parameter within about 5 seconds of beginning serial data communication, DTC U1255 is set. When more than one critical parameter does not have an identification number associated with it, the DTC 1255 will only be reported once.

CONDITIONS FOR RUNNING THE DTC
^ Voltage supplied to the module is in the normal operating voltage range (approximately 9 to 16 volts).
^ Diagnostic trouble codes U1300 and U1301 do not have a current status.
^ Module operation requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
At least one critical operating parameter has not been associated with an identification number within approximately 5 seconds after beginning serial data communication.

ACTION TAKEN WHEN THE DTC SETS
The module uses a default value for the missing parameter.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC U1255 will clear when all of the critical operating parameters for the module have been associated with an identification number or when the current ignition cycle ends.
^ A history DTC U1255 will clear upon executing 100 ignition RUN cycles with no recurrence of the malfunction.

DIAGNOSTIC AIDS
When a malfunction (such as an open fuse to a module) occurs while modules are communicating, a Lost Communication DTC is set as a current DTC. When the modules stop communicating (ignition is turned OFF) the current Lost Communication DTC is cleared but the history DTC remains.

When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules so DTC U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only DTC U1255 is set.

TEST DESCRIPTION

Steps 1-11:

Steps 12-16:

The number(s) below refer to the step number(s) on the diagnostic table.
1. A Lost Communication with XXX DTC with a history status may indicate the cause of U1255.
2. The module(s) which is not communicating is the likely cause of U1255 being set. The following modules communicate on the class 2 serial data circuit:
^ Body Control Module (BCM)
^ Integrated Radio Chassis (IRC)
^ Radio Interface Module (RIM)
^ Cellular Telephone
^ OnStar(R) VIU (W/W8Y).
5. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
6. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
7. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
11. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
13. If all modules are communicating, the module which set U1255 may have done so due to some other condition.
15. In some instances when a current U1255 is set and cannot be cleared using the scan tool, fuse V1 needs to removed in order to clear this DTC.

For a description of the RPO Code(s) shown in this article/images refer to the RPO Code List found at Vehicle/Application ID RPO Codes