U1128

Diagnostic Trouble Code (DTC) Chart:

Wiring Schematic:

CIRCUIT DESCRIPTION
Modules connected to the Class 2 data link monitor the link 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 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 U1128 is set.

CONDITIONS FOR RUNNING THE DTC
^ The voltage supplied to the module is in the normal operating voltage range (approximately 9-16 volts).
^ DTC U1300 and DTC U1301 do not have a current status.
^ 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 5 seconds.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC U1128 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 U1128 will clear upon receipt of a scan tool CLEAR DTCs command.

TEST DESCRIPTION
The number(s) below refer to the step number(s) on the diagnostic table.
1. A module which loses power during an ignition cycle will cause other module(s) to set Lost Communication DTCs.
2. This step inspects for a poor connection to the radio receiver preventing serial communication.
3. The module which was not communicating due to an open in the Class 2 serial data circuit may have Lost Communication DTCs for those modules that it was monitoring.
9. The module which can communicate, BCM and RCDLR (if equipped) indicates the module (the Radio) which cannot communicate. It is necessary to clear the DTCs from these modules to avoid future misdiagnosis.