The diagnostic procedure assumes that all modules are powered. A module with an internal Class 2 circuit malfunction or which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules.

Modules connected to the Class 2 serial 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 the module use a default value for that parameter. Once an identification number is learned by the module, it will monitor for that module's Node Alive message. Each module on the class 2 serial data link 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 five seconds, a DTC U1XXX (where xxx is equal to the 3-digit identification number) is set.

Voltage supplied to the module is in the normal operating voltage range (9.0-16.0 volts). A message from a learned identification number has not been detected for the past five seconds.

A current DTC U1xxx (where xxx is equal to the 3 digit identification number) will clear when a Node Alive message from the failed identification number is detected on the Class 2 serial data link or at the end of the current ignition cycle. A history DTC U1xxx will clear upon receipt of a scan tool Clear DTCs command.

When only DTCs which begin with a U are set with a history status, clear the DTCs. Otherwise, diagnose the malfunction using the appropriate diagnostic table.

The number(s) below refer to the step number(s) on the diagnostic table.

2. This step verifies that the module which does not communicate is powered and will not communicate via the Class 2 serial data link.

3. This test determines if the malfunction is due to the Class 2 serial data circuit or due to a malfunctioning control module.

4. This test determines if the malfunction is due to the Class 2 serial data circuit or due to a malfunction in splice pack SP201

6. This test inspects for an improper connection to SP201 that is preventing Class 2 serial communication.

8. The module which did not communicate due to a Class 2 serial data circuit malfunction will have set Lost Communication DTCs for those modules that it was monitoring.

10. In order to prevent future misdiagnosis, it is necessary to clear the DTC from the modules which can communicate because these modules indicate the modules which cannot communicate.