U2099

DTC U2099

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using the diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor
DTC U2099 00
- High Speed Communication Enable Circuit

Diagnostic Fault Information






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:

* Drive motor generator battery control module (BECM) (RPO HP2)
* Electronic brake control module (EBCM)
* Electronic suspension control (ESC) module
* Fuel pump flow control module (FPCM)
* Power steering control module (PSCM) (RPO HP2)

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 module(s) is never signaled, therefore the specific subsystem(s) 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

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

Reference Information
Schematic Reference

* Body Control System Schematics [1][2]Body Control Systems
* Data Communication Schematics Electrical Diagrams

Connector End View Reference

Component Connector End Views Connector Views
Description and Operation

Data Link Communications Description and Operation Description and Operation
Electrical Information Reference

* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs

Scan Tool Reference

Control Module References Control Module References for scan tool information.

Circuit/System Testing

Danger: Always perform the High Voltage Disabling procedure prior to servicing any High Voltage component or connection. Personal Protection Equipment (PPE) and proper procedures must be followed.


The High Voltage Disabling procedure will perform the following tasks:


* Identify how to disable high voltage.

* Identify how to test for the presence of high voltage.

* Identify condition under which high voltage is always present and personal protection equipment (PPE) and proper procedures must be followed.


Failure to follow the procedures exactly as written may result in serious injury or death.


1. With the ignition OFF, disconnect the module harness connectors listed below , if equipped, one at a time. After each disconnect, retest the BCM for current DTCs. DTC U2099 should remain current.

* Drive motor generator battery control module (BECM) (RPO HP2) - X 1
* Electronic brake control module (EBCM) - X 1
* Electronic suspension control (ESC) module - X 1
* Fuel pump flow control module (FPCM) - X 1
* Power steering control module (PSCM) (RPO HP2) - X 1

• If the DTC becomes history, replace the module disconnected before the DTC became history.

Important: If the vehicle is not equipped with RPO HP2 proceed to step 6.


2. Perform the High Voltage Disabling High Voltage Disabling procedure.
3. Ignition OFF, disconnect harness connector X1 of the BECM.
4. Reconnect the 12-volt battery.
5. Test the BCM for current DTCs. DTC U2099 should remain current.

• If the DTC becomes history, replace the BECM.

6. Ignition OFF, disconnect the harness connector X 3 at the BCM.
7. With the above harness connectors still disconnected, test for infinite resistance between the high speed GMLAN serial data wake up circuit terminal 19 and ground.

• If less than the specified value, test the wake up circuit for a short to ground.

8. If all circuits test normal and DTC is current, replace the BCM.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.

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