For 1990-2009 cars

### 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, it will monitor for that module's 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 2 seconds. When no message is detected from a learned identification number for 10 seconds, a DTC U1xxx (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 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.

Control Module | ID Number
--- | ---
Powertrain Control Module (PCM) | 016
Electronic Brake Control Module (EBCM) | 040
Body Control Module (BCM) | 064
Sensing and Diagnostic Module (SDM) | 088
Instrument Panel Cluster (IPC) | 096

When more than one Loss of Communication DTC is set in either one module or in several modules, diagnose the DTCs in the following order:

1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
2. The DTC which is reported the most times.
3. From the lowest number DTC to the highest number DTC.

### 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 ten 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 class 2 serial data circuit or at the end of the current ignition cycle.

• | 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 class 2 serial data circuit. Use the Control Module ID Number list in order to determine which modules are not communicating. Use the class 2 serial data circuit schematic in order to determine the location of the opens. Refer to Data Link Connector Schematics .

### Test Description

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

1. The module which was not communicating due to an open in the class 2 serial data circuit may have set Loss of Communication DTCs for those modules that it was monitoring.

2. The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs from these modules to avoid further misdiagnosis.

Step | Action | Yes | No
--- | --- | --- | ---
1 | Test the battery positive voltage circuit of the the module that is not communicating for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. For the applicable schematic, refer to Control Module References .

Did you find and correct the condition?

Go to Step 6

Go to Step 2

2 |

1. Turn OFF the ignition.
2. Test the ground circuit of the module that is not communicating for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. For the applicable schematic, refer to Control Module References .

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 class 2 serial data circuit of the harness connector 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 6

Go to Step 4

4 | Test the class 2 serial data circuit of the module that is not communicating for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. For the applicable schematic, refer to Control Module References . Did you find and correct the condition? | Go to Step 6 | Go to Step 5
5 | Replace the module that is not communicating. Refer to Control Module References for the applicable repair procedure. Did you complete the replacement? | Go to Step 6 | --
6 |

1. Install a scan tool.
2. Turn ON the ignition leaving the ignition OFF.
3. Select the Display DTCs function for the module that was not communicating.

Does the scan tool display any DTCs that do not begin with a U?

Go to Control Module References for 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 Control Module References for the applicable Diagnostic System Check | Go to Step 9
9 |

1. Use the scan tool in order to clear the DTCs.
2. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.

Did you complete the action?

System OK

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