2019-2025 Audi MHEV Electrical Fault U046900 TSB

Audi Technical Service Bulletin (TSB) 2073284/2, issued on June 5, 2024, provides critical information regarding an Audi MHEV electrical fault affecting various 2019-2025 Audi models. This bulletin supersedes TSB Group 27 number 24-68 dated March 9, 2024, and focuses on diagnosing and resolving an electrical system fault related to the 48V Mild Hybrid Electric Vehicle (MHEV) system.

This issue can manifest as a warning in the instrument cluster and, if left unaddressed, may lead to the vehicle losing power due to excessive battery discharge. Understanding the root cause and proper repair procedure is crucial for owners of affected vehicles.

Symptoms Owners May Notice

Owners of 2019-2025 Audi A6, A7, A8, Q7, and Q8 models may experience an "Electrical system fault" message appearing in the instrument cluster. This warning indicates a problem within the vehicle's electrical system, specifically related to the 48V MHEV components. A significant risk associated with this fault is the potential for the vehicle to lose power if the battery becomes severely discharged while driving.

Fault Codes / DTCs

Technicians may find one or more of the following Diagnostic Trouble Codes (DTCs) stored in the Engine Electronics (-J623-, address word 0001) or Brake Electronics (-J104-, address word 0003) control modules. For DTC U046900, the error type is often recorded as "passive/sporadic" with a status of "00100000" and symptom code 34763.

Code	Description	Module	Symptom
U046900	Lost Communication with Starter/Generator Control Module	Engine Electronics (0001)	34763
U002900	Sensor/Hybrid CAN No Communication	Engine Electronics (0001)
U002800	Sensor/Hybrid CAN Faulty	Engine Electronics (0001)
C054A00	Brake Servo Pressure Sensor	Brake Electronics (0003)
C11EC04	Brake Servo Pressure Sensor	Brake Electronics (0003)

Important Warnings and What Not To Do

This TSB highlights critical information to prevent misdiagnosis and incorrect repairs, ensuring the Audi MHEV electrical fault is addressed correctly:

• Do not replace the 48V Starter/Generator. The bulletin explicitly states that replacing this component will not resolve the problem. The DTC U046900, indicating "Lost Communication with Starter/Generator Control Module," is a symptom of another underlying electrical system issue on the vehicle's CAN bus, not a fault of the Starter/Generator itself. The technical background notes that this is often documented in event memory with the value "Starter generator, actual mode 48V = 8".
• Do not assume a faulty Starter/Generator if it cannot be reached in ODIS. If the Starter Generator (diagnostic address 00CC) cannot be accessed via ODIS (Offboard Diagnostic Information System), this does not automatically mean the component is defective.
• Warranty Information: This Technical Service Bulletin is informational only and is not applicable to any Audi Warranty.

Technical Background and Root Cause

The core of the Audi MHEV electrical fault lies not with the 48V Starter/Generator itself, but with other electrical system problems, particularly on the vehicle's CAN bus. The Starter/Generator is designed to freewheel when the Engine Electronics commands it. This can happen if sensor faults in the Engine Electronics or Brake Electronics, or communication errors, influence the vehicle data bus system.

As a result, the Engine Electronics commands the 48V Starter/Generator to freewheel, and the electrical system faults may then occur because the battery will not be charged. The TSB includes Figure 1, which illustrates the 'Starter generator, actual mode 8' condition, and Figure 2, which depicts various connector failures that can contribute to these issues.

Diagnosis and Repair Focus

To properly address the Audi MHEV electrical fault, technicians must process the sensor faults in the Engine Electronics, Brake Electronics, and any communication error faults to find the root cause. Specifically, fault codes U002900 ("Sensor/Hybrid CAN No Communication") and U002800 ("Sensor/Hybrid CAN Faulty") stored in diagnostic address 0001 (Engine Electronics) point to several potential causes:

• Communication Connectors: These may not have been plugged in or fastened correctly. Pins within the connectors could be failing (bent or widened), or there might be short circuits in the associated wiring.
• Incorrect Connector Swaps: Issues can arise from incorrect attempts to swap connectors, such as a failure to connect vehicle ground cables.

Therefore, diagnosis should focus on inspecting these areas rather than immediately replacing the Starter/Generator.

What Owners Should Do

If you experience an "Electrical system fault" message in your instrument cluster, it is important to have your vehicle inspected by an authorized Audi service center. Do not attempt to replace the 48V Starter/Generator yourself, as this TSB indicates it will not resolve the underlying problem. An Audi technician will perform diagnostics to identify and correct sensor faults, communication errors, or connector issues on the CAN bus that are causing the Audi MHEV electrical fault.

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Source: NHTSA ID 11002962