U0028
DTC U0028
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this 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 U0028
- MOST Bus
For symptom byte information, refer to Symptom Byte List Symptom Byte List.
Diagnostic Fault Information
Circuit/System Description
The Media Oriented Systems Transport (MOST) Infotainment network is a dedicated high speed multimedia streaming data bus independent from GMLAN. The MOST bus will be configured in a physical hardwired loop with each device within the bus sends and receives data on an assigned MOST addresses in a set order. Each device on the MOST bus will be required to have twisted pair copper wires (2 transmit TX, 2 receive RX, and 1 electronic control line which is a 12 V wake up signal line). The A11 Radio is the MOST Master and will monitor the bus for vehicle configuration, Infotainment data messages and errors on the bus. The MOST initialization consists of a short 100 ms low voltage pulse on the electronic control line connected to all devices contained on the MOST ring. When the MOST devices receive this wakeup message, they will respond with a generic device response. Once these initial responses on the MOST bus are reported successfully without error to the A11 Radio, the second data request will record the device MOST device addresses, their functionality requirements and capabilities within. The A11 Radio will learn this information and also record the address node sequence on the MOST bus at this point. This node address list will now be stored within the A11 Radio as the MOST bus configuration (called "Last Working MOST ID of Node 1-9" on scan tool data display).
When MOST receive, transmit, or control line faults are detected, transmit/receive messages will not received as expected from the wakeup request. The A11 Radio and the K74 Human Machine Interface Control Module will then perform diagnostics to isolate these MOST faults. If the MOST control line is shorted low to 0 V for excess amount of time, the A11 Radio will set a U2098 DTC and K74 Human Machine Interface Control Module will set a U0029 02 DTC. At this point the MOST bus will be unable to communicate until the shorted MOST control line is repaired.
Once the shorted MOST control line diagnostics pass, the A11 Radio will attempt to resend the initial short pulse attempts up to 3 times on the MOST control line. If the expected responses are not received, the A11 Radio continues into a failure mode setting a U0028 DTC and will continue on to send one 300 ms long pulse, which will enable the furthest upstream transmitting device to become the surrogate MOST Master in this MOST fault/diagnostic mode. When the A11 Radio receives this new MOST Master identity, the surrogate MOST master device can be identified based on scan tool data parameter "Surrogate MOST Master Node Upstream Position". The scan tool should be used to determine the MOST bus configuration and direction by utilizing the "Last Working MOST ID of Node 1 - 9" parameters from the A11 Radio data display. When a fault is present, it will indicate the newly enabled "Surrogate MOST Master Node Upstream Position" from the A11 Radio. This will assist in determining where the MOST bus/control is at fault. The MOST device upstream from the surrogate MOST master device, transmit, receive, or control lines will be the suspect areas for diagnostics at this point. These faults can be associated with any of the MOST transmit, receive, or control line twisted copper wires or possibly an internal device fault.
Conditions for Running the DTC
* The system voltage is between 9-16 V.
* The ignition is ON.
* The Radio is ON.
Conditions for Setting the DTC
A fault occurs on the transmit, receive, or electronic control lines.
Action Taken When the DTC Sets
Some or all of the infotainment system may be inoperative.
Conditions for Clearing the DTC
The MOST initialization is successful without errors and the MOST bus is locked.
Diagnostic Aids
* Use A11 Radio's scan tool data display to determine the Surrogate MOST Master by interpreting the "Surrogate MOST Master Node Upstream Position" parameter and determine the MOST bus configuration from "Last Working MOST ID of Node 1-9" parameters.
* If the MOST bus configuration is A11 Radio > K74 Human Machine Interface Control Module > T3 Audio Amplifier > P16 Instrument Cluster > A33 Media Disc Player and back to A11 Radio. The A11 Radio's scan tool data display will be shown as followed:
- Last Working MOST ID of Node 1: Radio
- Last Working MOST ID of Node 2: Human Machine Interface Control Module
- Last Working MOST ID of Node 3: Audio Amplifier
- Last Working MOST ID of Node 4: Instrument Cluster
- Last Working MOST ID of Node 5: Media Disc Player
* After a ring break diagnostic session, the A11 Radio, which is the MOST bus master, indicates a ring break was detected and sets DTC U0028. The "Surrogate MOST Master Node Upstream Position" parameter will now be available in the A11 Radio's scan tool data display. Starting from the bottom of the MOST device list, count upward to determine the surrogate MOST Master device. The MOST ring break will be found upstream from the surrogate MOST Master since this will be the node which timed out and initiated the MOST node assignments during the MOST ring break.
- If the Surrogate MOST Master Node Upstream Position value is 0, the Surrogate MOST Master is also the A11 Radio. The MOST ring break is either at the A33 Media Disc Player or between the A11 Radio and between the A33 Media Disc Player.
- If the Surrogate MOST Master Node Upstream Position value is 1, the Surrogate MOST Master is the A33 Media Disc Player which is 1 node upstream from the A11 Radio. The MOST ring break is either at the P16 Instrument Cluster or between the A33 Media Disc Player and the P16 Instrument Cluster.
- If the Surrogate MOST Master Node Upstream Position value is 2, the Surrogate MOST Master is the P16 Instrument Cluster which is 2 nodes upstream from the A11 Radio. The MOST ring break is either at the T3 Audio Amplifier or between the P16 Instrument Cluster and the T3 Audio Amplifier.
- If the Surrogate MOST Master Node Upstream Position value is 3, the Surrogate MOST Master is the T3 Audio Amplifier which is 3 nodes upstream from the A11 Radio. The MOST ring break is either at the K74 Human Machine Interface Control Module or between the T3 Audio Amplifier and the K74 Human Machine Interface Control Module.
- If the Surrogate MOST Master Node Upstream Position value is 4, the Surrogate MOST Master is the K74 Human Machine Interface Control Module which is 4 nodes upstream from the A11 Radio. The MOST ring break is between the K74 Human Machine Interface Control Module and the A11 Radio.
* In the case of a valid optional device being installed, DTC U0028 may set because of the change in the node assignment list. Be sure to clear the DTC after the installation is finished.
* An open in the MOST control line or a fault in the MOST transmit/receive lines will cause DTC U0028 to be set.
Reference Information
Schematic Reference
* Data Communication Schematics [1][2]Electrical Diagrams
* Control Module References Control Module References
Connector End View Reference
Component Connector End Views Fuel Pump and Level Sensor Assembly
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 Verification
1. Ignition ON. Radio ON.
2. Verify DTC U0028 is set.
• If the DTC is set
1. Access the A11 Radio's Last Working MOST ID of Node 1-9 parameters with a scan tool to view MOST device location IDs and MOST bus configuration.
2. Access the A11 Radio's Surrogate MOST Master Node Upstream Position parameter with a scan tool to determine the surrogate MOST Master device and where the MOST ring break occurs. For more information, refer to Diagnostic Aids section.
3. Refer to Circuit/System Testing.
• If the DTC is not set
3. All OK.
Circuit/System Testing
Note: The MOST devices on both sides of the MOST ring break point will need to be disconnected to isolate a circuit fault.
Use the schematic to identify the following:
* The MOST bus configuration and MOST device locations on the MOST serial data circuits
* Each MOST device's electronic control line and serial data circuit terminals
1. Ignition OFF, disconnect the harness connectors at the MOST device upstream from the surrogate MOST Master device where the MOST ring break occurs, ignition ON.
2. Verify a test lamp illuminates between the MOST control circuit at the MOST device connectors that was just disconnected and ground.
• If the test lamp does not illuminate
1. Ignition OFF, disconnect the harness connectors at the A11 Radio.
2. Test for infinite resistance between the MOST control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 ohms in the MOST control circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the A11 Radio.
• If the test lamp illuminates
3. Ignition OFF, disconnect the harness connectors at the surrogate MOST Master device, ignition ON. The following tests will be performed on the MOST serial data circuits between the two MOST devices that were just disconnected.
4. Test for less than 1 V between each MOST serial data circuit and ground.
• If any serial data circuit are 1 V or greater
Repair the short to voltage in the circuit.
• If each serial data circuit are less than 1 V
5. Ignition OFF, test for infinite resistance between each MOST serial data circuit and ground.
• If any serial data circuit are less than infinite resistance
Repair the short to ground in the circuit.
• If each serial data circuit are infinite resistance
6. Test for infinite resistance between each pair of the MOST serial data circuits.
• If any pair of serial data circuits are less than infinite resistance
Repair a short together in the circuit.
• If each pair of serial data circuits are infinite resistance
7. Test for less than 2 ohms in each MOST serial data circuit end to end.
• If any serial data circuit are 2 ohms or greater
Repair the open/high resistance in the circuit between the two MOST devices.
• If each serial data circuit are less than 2 ohms
8. Replace the MOST device upstream from the surrogate MOST Master device where the MOST ring break occurs.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the repair.
* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Control Module References for device replacement, programming and setup