For 1990-2009 cars

### Circuit Description

Modules connected to the GMLAN serial data circuit monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. The body control module (BCM) maintains a transmit error counter (TEC) and a receive error counter (REC). The counter values increase with detected errors and will decrease with error free messages.

### DTC Descriptors

This diagnostic procedure supports the following DTCs:

• | DTC U2105 Lost Communication with ECM

• | DTC U2106 Lost Communication with TCM

• | DTC U2142 Lost Communication with Sensor Cluster

• | DTC U2143 Lost Communication with Steering Wheel Angle Sensor

### Conditions for Running the DTC

• | Voltage supplied to the module is in the normal operating voltage range.

• | The vehicle power mode requires serial data communication to occur.

### Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received.

### Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

### 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.

### Diagnostic Aids

• | A poor connection at the inoperative module may cause this code to set.

• | An improperly powered module may cause this code to set.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics or Body Control System Schematics Connector End View Reference: Component Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Important: Use the DTC List in DTC Descriptors to determine which is the module that is not communicating. Test the following circuits of the module that is not communicating for an open or a short to ground:

• | The battery positive voltage input circuits

• | The battery positive voltage output circuits

• | The ignition voltage input circuits

• | The ignition voltage output circuits

• | The switched battery positive voltage circuits

Did you find and correct the condition?

Go to Step 7

Go to Step 3

3 |

1. Turn OFF the ignition.
2. Test the ground circuits of the module that is not communicating for an open. If the module is the BCM, test its class 2 serial data circuit for an open, also.

Did you find and correct the condition?

Go to Step 7

Go to Step 4

4 | Test the high speed GMLAN serial data circuits of the module that is not communicating for an open. Did you find and correct the condition? | Go to Step 7 | Go to Step 5
5 | Inspect the harness connectors of the module that is not communicating for poor connections and terminal tension at the following circuits:

• | The battery positive voltage input circuits

• | The battery positive voltage output circuits

• | The ignition voltage input circuits

• | The ignition voltage output circuits

• | The switched battery positive voltage circuits

• | The ground circuits

• | The high speed GMLAN serial data circuits

• | The class 2 serial data circuit, if the modules is the BCM

Did you find and correct the condition?

Go to Step 7

Go to Step 6

6 | Replace the module that is not communicating. Did you complete the replacement? | Go to Step 7 | --
7 | Use the scan tool in order to clear the DTCs. Does the scan tool still display other current DTCs? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK