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 Node Alive messages. 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 5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

CONDITIONS FOR RUNNING THE DTC
^ The following diagnostic trouble codes do not have a current status:
^ B1982
^ B1983
^ U1300
^ U1301
^ U1305
^ The vehicle power mode 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 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 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
When a malfunction such as an open fuse to a module occurs while modules are communicating, a Loss of U1XXX Communication DTC is set current. When the modules stop communicating the current Loss of U1XXX 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 U1000 or U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 is set.

TEST DESCRIPTION

Steps 1-7:

Steps 8-14:

The numbers below refer to the step numbers on the diagnostic table.
1. A Loss of U1XXX Communication DTC U1001-U1254 with a history status may indicate the cause of U1000 or U1255.
2. The modules not communicating is the likely cause of U1000 or U1255. The following modules communicate on the class 2 serial data circuit:
^ Body control module (BCM)
^ Continuously variable road sensing suspension (CVRSS) module w/F45
^ Electronic brake control module (EBCM)
^ HVAC control module
^ HVAC control processor
^ Inflatable restraint sensing and diagnostic module (SDM)
^ Instrument panel cluster (IPC)
^ Memory mirror module (MMM) w/A45
^ Memory seat module (MSM) w/A45
^ Powertrain control module (PCM)
^ Radio
^ Radio interface module (RIM) w/UQ4
^ Remote control door lock receiver (RCDLR)
^ Vehicle interface unit (VIU)
7. The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set Loss of XXX Communication DTCs U1001-U1254 for those modules that it was monitoring.
9. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
11. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
12. The module which set U1000 or U1255 is the likely cause of the malfunction.

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