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 . These messages are also known as 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 which is powered and performing functions that require detection of a communications malfunction is required to send a node alive message every two seconds. When no message is detected from a learned identification number for five seconds, a DTC U1xxx is set.

The control module ID number list provides a method for determining which module is not communicating. A module with an internal class 2 serial data circuit malfunction or which loses power during the current ignition cycle would have a lost communication DTC set by other modules. Use the control module ID number list in order to determine which module is not communicating and the DTC U1001-U1254 diagnostic table in order to diagnose the malfunction.

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

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

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

6. The module which was not communicating due to an open in the class 2 serial data circuit 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 DTC from these modules to avoid future misdiagnosis.