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 U2099 00: High Speed Communication Enable Circuit

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
High Speed GMLAN Serial Data Wake Up | U2099, U0100-U0299¹ | U0100-U0299 | U0100-U0299² | --
¹ Vehicle will not start. ² May be history only. If cleared the DTC should return after several ignition cycles.

### Circuit/System Description

The body control module (BCM) applies voltage to the high speed GMLAN serial data wake up circuit, when the ignition key is in ACC, ON or START to enable several modules for bus communication.

Depending on option content, modules that may receive the signal are as follows:

• | Electronic brake control module (EBCM)

• | Electronic suspension control (ESC) module - (FE1) (F38) (F55)

• | Distance sensing cruise control (DSCC) module - (K59)

### Conditions for Running the DTC

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

• | The BCM attempts to enable the modules connected to the high speed GMLAN serial data wake up circuit.

### Conditions for Setting the DTC

The BCM senses a short to ground on the high speed GMLAN serial data wake up circuit.

### Action Taken When the DTC Sets

• | The output command is turned off while the malfunction is present.

• | The modules use a default value for the missing parameters.

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

• | The vehicle will not start while the circuit is shorted to ground.

### 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 DMM MIN/MAX function to capture/locate intermittent conditions.

• | Short to ground will also set multiple history no communication codes for GMLAN high speed modules

### Reference Information

#### Schematic Reference

• | Body Control System Schematics

• | Data Communication Schematics

#### Connector End View Reference

Master Electrical Component List

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

### Circuit/System Testing

1. With the ignition OFF, disconnect the harness connector at the EBCM, ESC module, and DSCC module one at a time. After each disconnect, clear the DTC, cycle the ignition key 3 times, then retest for the DTC. DTC U2099 should reset.

⇒ | If the DTC does not reset, replace the last module disconnected.

2. Ignition OFF, disconnect the harness connector at the BCM.
3. With all modules disconnected, test for infinite resistance between the high speed GMLAN serial data wake up circuit and ground.

⇒ | If less than infinite resistance, repair the wake up circuit for a short to ground.

4. If all tests normal and DTC is current, replace the BCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Refer to Control Module References for module replacement, setup, and programming.