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 Descriptors

DTC U2105 00: Lost Communication With Engine Control System
DTC U2106 00: Lost Communications With Transmission Control System
DTC U2107 00: Lost Communications With Body Control System
DTC U2142 00: Lost Communications With Yaw Rate Sensor
DTC U2143 00: Lost Communications With Steering Angle Sensor

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Module Voltage Circuits (input or output) | U2105-U2199 | U2105-U2199 | -- | --
Module Ground Circuits | -- | U2105-U2199 | -- | --

### Circuit/System Description

Modules connected to the high 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 prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module.

### Conditions for Running the DTC

• | The system voltage is between 9-16 volts.

• | The vehicle power mode master 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 modules is never signaled, therefore the specific subsystems will not function.

### 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 of 50, without a repeat of the malfunction.

### Diagnostic Aids

• | Use the DTC Descriptor list above to determine the module which is not communicating.

• | If there are multiple non-communicating modules choose the one closest to the data link connector (DLC).

• | Some modules may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no module output voltage circuit is shorted to ground before replacing the module.

### Reference Information

#### Schematic Reference

• | Data Communication Schematics

• | Control Module References

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

### Circuit/System Testing

1. Using the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector of the module that is not communicating.
3. Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground.

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

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

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

5. Test for less than 1 ohm between the module ground circuits and ground.

⇒ | If greater than 1 ohm, repair the ground circuit for an open.

6. If all circuits test normal, replace the module that is not communicating.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | GMLAN Wiring Repairs

• | Control Module References for module replacement, setup, and programming