Circuit Description
Modules connected to the UART 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 module's Node Alive message. Each module on the UART 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 B1xxx (where xxx is equal to the three digit identification number) is set.
The control module ID number list below provides a method for determining which module is not communicating. A module with a UART 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.
Control Module | ID Number |
|---|---|
Powertrain Control Module (PCM) | 021 |
Supplemental Inflatable Restraint (SIR) Module | 022 |
Electronic Brake Control Module (EBCM) | 023 |
Conditions for Running the DTC
The 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 five seconds.
Conditions for Clearing the DTC
| • | A current DTC will clear when a Node Alive message from the failed identification number is detected on the UART serial data circuit or at the end of the current ignition cycle. |
| • | A history DTC will clear after 100 ignition ON/OFF cycles with no repeated failure. |
| • | A history DTC will clear upon receipt of a scan tool Clear DTCs command. |
Diagnostic Aids
When multiple Loss of Communication DTCs are set concurrently, the cause is likely to be two opens in the UART serial data circuit. Use the Control Module ID Number list in order to determine which modules are not communicating. Use the UART serial data circuit information in the schematics in order to determine the location of the opens. Refer to Instrument Cluster: Digital Schematics .
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Yes | No |
|---|---|---|---|
1 | Test the battery positive voltage circuit of the module that is not communicating for an open or a short to ground. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems. For the applicable schematic, refer to Engine Controls Schematics in Engine Controls - 2.2 L, Engine Controls Schematics in Engine Controls - 2.4 L, SIR Schematics in SIR or ABS/ETS Schematics in ABS. Did you find and correct the condition? | Go to Step 6 | Go to Step 2 |
2 |
Did you find and correct the condition? | Go to Step 6 | Go to Step 3 |
3 | Inspect for poor connections at the battery positive voltage circuit, the ground circuit, and the UART serial data circuit(s) of the harness connector of the module that is not communicating. Refer to Intermittents and Poor Connections Diagnosis , Connector Repairs and Repairing Connector Terminals in Wiring Systems. Did you find and correct the condition? | Go to Step 6 | Go to Step 4 |
4 | Test the UART serial data circuit of the module that is not communicating for an open. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems. For the applicable schematic, refer to Engine Controls Schematics in Engine Controls-2.2 L, Engine Controls Schematics in Engine Controls-2.4 L, SIR Schematics in SIR or ABS/ETS Schematics in ABS. Did you find and correct the condition? | Go to Step 6 | Go to Step 5 |
|
Important: Program the replacement module, if applicable Replace the module that is not communicating. Refer to Powertrain Control Module Replacement in Engine Controls 2.2 L, Powertrain Control Module Replacement in Engine Controls 2.4 L, Inflatable Restraint Sensing and Diagnostic Module Replacement in SIR or Electronic Brake Control Module Replacement in ABS. Did you complete the replacement? | Go to Step 6 | -- | |
6 |
Does the scan tool display any DTCs that do not indicate a communications failure? | Go to the applicable Diagnostic System Check | Go to Step 7 |
7 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 8 | -- |
8 | Select the Display DTCs function for the module(s) which had the Loss of Communications DTC set. Does the scan tool display any DTCs which do not begin with a U? | Go to the applicable Diagnostic System Check | Go to Step 9 |
9 |
Did you complete the action? | System OK | -- |
