U2105
DTC U2105-U2199
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
Refer to Control Module U Code List (Control Module U Code List).
Diagnostic Fault Information
Circuit/System Description
Control modules connected to the high and low speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control 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.
The body control module (BCM) activates the high speed GMLAN serial data wake up and the accessory wake up serial data circuits by applying voltage when the ignition key is in ACC, ON or START. The high speed GMLAN serial data wake up circuit wakes up the electronic brake control module (EBCM) for bus communication and the accessory wake up serial data circuit wakes up the engine control module (ECM) and transmission control module (TCM).
Conditions for Running the DTC
* The system voltage is between 9-16 volts.
* 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 control module uses a default value for the missing parameters.
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).
* Use Data Link References (Initial Inspection and Diagnostic Overview) to determine if the control module uses high or low speed GMLAN serial data communications
* Some control 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 control module output voltage circuit is shorted to ground before replacing the control module.
* This diagnostic can be used for any control module that should communicate with high or low speed GMLAN serial data providing the vehicle is equipped with the option that uses that control module.
Reference Information
* Data Communication Schematics (Electrical Diagrams)
* Control Module References (Programming and Relearning)
Connector End View Reference
Component Connector End Views (Connector End Views By Name)
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 References
Control Module References (Programming and Relearning) for scan tool information
Circuit/System Testing
Note: If more than one control module is not communicating use Data Communication Schematics (Electrical Diagrams) to determine which control module is closest to the DLC. Start diagnostics with that control module.
Terminal and connector identifiers will be different depending on the individual control module being tested. Use Schematic Reference and Connector End View Reference to determine which connector and terminal to access.
1. Using the DTC Descriptor and Diagnostic Aids, determine the control module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector at the control module that is not communicating.
3. Ignition OFF, test for less than 2 ohms between each ground circuit terminal and ground.
• If greater than specified range, test the ground circuit for an open/high resistance.
Note: Only the high speed GMLAN modules have a wired serial data wake up circuit. Refer to the module schematics to identify which control modules have a wake up circuit. If the control module that is not communicating does not have a serial data wake up circuit, proceed to step 5.
4. Ignition ON, verify that a test lamp illuminates between the high speed GMLAN serial data wake up circuit OR the accessory wake up serial data circuit terminal and ground.
• If the test lamp does not illuminate, test the wake up circuit for an open/high resistance. If the circuits test normal, replace the K9 body control module (BCM).
5. Ignition ON, verify that a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate, test the B+ 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 control module for a short to ground. If the circuits test normal, replace the appropriate control module.
6. Ignition ON, verify that a test lamp illuminates between each ignition voltage circuit terminal and ground.
• If the test lamp does not illuminate, test the ignition voltage 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 control module for a short to ground. If the circuits test normal, replace the appropriate control module.
7. Ignition OFF for 60 seconds, test for less than 1 Ohms in each GMLAN serial data circuit between the non-communicating control module and the control module that displays the DTC.
• If greater than specified range, test the GMLAN serial data circuit for open/high resistance.
8. If all circuits test normal, replace the control module that is not communicating.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* GMLAN Wiring Repairs (GMLAN Wiring Repairs)
* Control Module References (Programming and Relearning) for module replacement, setup, and programming