Circuit Description
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, the module will monitor for the sending module's "Node Alive" message. Each module on the class 2 serial data circuit which is performing functions that require detection of a communication 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 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. 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 5 seconds, a DTC U1xxx, where xxx is equal to the 3 digit identification number is set.
Control Module | ID Number |
|---|---|
PCM | 016 |
EBCM | 040 |
BCM | 064 |
SDM | 088 |
IPC | 096 |
Radio | 128 |
OnStar | 151 |
HVAC | 153 |
When more than 1 Loss of Communication DTC is set in either one module or several modules, diagnose the DTCs in the following order:
- Current DTCs before history DTCs, unless told otherwise in the diagnostic tables
- The DTC which is reported the most times
- From the lowest number DTC to the highest number DTC
Conditions for Running the DTC
| • | Voltage supplied to the module is in the normal operating voltage range (approximately 9 to 16 volts). |
| • | Diagnostic trouble codes U1300 and U1301 do not have a current status. |
| • | The module operation or 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 clears when the malfunction is no longer present. |
| • | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
A module which loses power supply during an ignition cycle will cause other module to set Lost Communication DTCs.
-
A module which loses ground during an ignition cycle will cause other module to set Lost Communication DTCs.
-
The malfunction is due to an open in the class 2 serial data circuit or an open in the module.
-
The module which was not communicating may have set Lost Communication DTCs for those modules that it was monitoring.
-
The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules in order to avoid future misdiagnosis.
Step | Action | Yes | No |
|---|---|---|---|
Test the battery positive voltage circuits and the ignition voltage circuits of the module that is not communicating for an open or a short to ground. Refer to Control Module References in Body Control System for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 9 | Go to Step 2 | |
Did you find and correct the condition? | Go to Step 9 | Go to Step 3 | |
Did you find and correct the condition? | Go to Step 7 | Go to Step 4 | |
4 | Test the class 2 serial data circuit of the module that is not communicating for an open between the module and the star connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 7 | Go to Step 5 |
5 | Inspect for poor connections at the battery positive voltage circuits, the ignition voltage circuits, the ground circuits, and the class 2 serial data circuit of the module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 7 | Go to Step 6 |
6 | Replace the module that is not communicating. Refer to Control Module References in Body Control System for the appropriate repair instructions. Did you complete the repair? | Go to Step 9 | -- |
Does the scan tool display any DTCs which do not begin with a "U"? | Go to Control Module References in Body Control System for the applicable Diagnostic System Check | Go to Step 8 | |
8 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 9 | -- |
Select the Display DTCs function for the modules which had the Lost Communication with xxx DTC set. Does the scan tool display any DTCs which do not begin with a "U"? | Go to Control Module References in Body Control System for the applicable Diagnostic System Check | Go to Step 10 | |
10 |
Did you complete the action? | System OK | -- |
