For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC U0100: Lost Communication With Engine/Powertrain Control Module (ECM/PCM)
DTC U0101: Lost Communication With Transmission Control Module (TCM)
DTC U0121: Lost Communication With Electronic Brake Control Module (EBCM)
DTC U0140: Lost Communication With Body Control Module (BCM)
DTC U0198: Lost Communication With Communication Interface Module (OnStar®)
DTC U2105: Lost Communication With Engine Control Module (ECM)
DTC U2106: Lost Communication With Transmission Control Module (TCM)
DTC U2107: Lost Communication With Body Control Module (BCM)
DTC U2108: Lost Communication With Electronic Brake Control Module (EBCM)
DTC U2113: Lost Communication With Sensing Diagnostic Module (SDM)
DTC U2122: Lost Communication With Radio
DTC U2125: Lost Communication With Mobile Communications Control Systems
DTC U2137: Lost Communication With Vehicle Theft Deterrent (VTD)
DTC U2172: Lost Communication With Digital Radio Receiver

### Circuit/System Description

Modules connected to the GMLAN serial data circuits monitor for serial data communications, during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have programmed information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module. When a message that indicates the availability of the transmitter module is not received, the receiver module sets a DTC against the transmitter module.

The DTC descriptors listed above provide a method for determining which module is not communicating. A module with a GMLAN 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 cannot communicate.

### Conditions for Running the DTC

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

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

• | The DTC U2100 does not have a current status.

### 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

Diagnose any power moding DTCs prior to diagnosing data communication DTCs.

### Reference Information

#### Schematic Reference

Data Communication Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool References

Control Module References for Scan Tool Information

### Circuit/System Testing

Important: If more than one module is not communicating , refer to Data Communication Schematics to determine which module is closest to the DLC. Start diagnostics with that module.

1. Ignition OFF, disconnect the harness connector of the module that is not communicating.
2. Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the module.

3. Verify that a test lamp illuminates between all ignition voltage circuits and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the module.

4. Ignition OFF, test for less than 5 ohm of resistance between the module ground circuits and ground.

⇒ | If greater than 5 ohm, test the circuit for an open/high resistance.

5. Ignition OFF, test for less than 5 ohm of resistance between the GMLAN serial data circuits of the non-communicating module and the module that displays the DTC.

⇒ | If greater than 5 ohm of resistance, test the GMLAN serial data circuits for open/high resistance.

6. If all circuits test normal, replace the non communicating module.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for module replacement, setup and programming