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 modules node alive message. Each module on the class 2 serial data circuit is required to send a Node Alive message every 2 seconds. When no message is detected from a learned identification number for 5-10 seconds, a DTC U1XXX. XXX is equal to the 3 digit identification number of the module.

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. The following modules communicate on class 2 serial data circuit:

The body control module (BCM) emulates the control modules that are on controller area network (CAN) serial data circuit to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of those CAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on CAN serial data circuit and are emulated by BCM on class 2 serial data circuit:

When more than one loss of communication DTC is set in a single module or among multiple modules, diagnose the DTCs in the following order:

1. Current DTCs before history DTCs except as specified 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 node alive message has not been received from an identified module within the last 5 seconds.

The module uses a default value for the missing parameter.

The number below refers to the step number on the diagnostic table.

7. The module which was not communicating on the class 2 serial data circuit may have set Loss of Communication DTCs for those modules that it was monitoring.