Modules connected to the 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 modules "Node Alive" message. Each module on the 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 provides a method for determining which module is not communicating. A module with an internal serial data circuit malfunction or which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules. The modules that can communicate will set a DTC indicating the module that cannot communicate. When no message is detected from a learned identification number for five seconds, a DTC U1xxx, where xxx is equal to the 3 digit identification number, is set.

This diagnostic procedure supports the following DTCs:

DTC U1XXX Lost Communication With XXX Module

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:

1. Current DTCs before history DTCs unless told otherwise in the diagnostic tables.
2. The DTC which is reported the most times.
3. From the lowest number DTC to the highest number DTC.

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

The numbers below refer to the step numbers on the diagnostic table.

1. A module which loses power during an ignition cycle will cause other modules to set Lost Communication DTCs.

2. A module which loses ground during an ignition cycle will cause other modules to set Lost Communication DTCs.

3. The malfunction is due to an open in the serial data circuit or an open in the module.

6. The module which was not communicating may have set Lost Communication DTCs for those modules that it was monitoring.

8. The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs from these modules to avoid future misdiagnosis.