2022-2026 Audi A3/S3/RS3 DTC U300100 Brake Servo Issue

Owners of 2022-2026 Audi A3, S3, and RS3 vehicles may encounter a diagnostic trouble code (DTC) U300100, which points to a problem with the electromechanical brake servo, identified as DA0023. This issue can manifest through various warning messages and indicator lights on the instrument cluster. This article details the symptoms, diagnosis, and repair procedure for the Audi S3 DTC U300100 as described in Audi's Technical Service Bulletin (TSB) 2079459/1.

Symptoms Owners May Notice

Drivers might observe several warning messages or indicator lights related to this issue. These messages appear in the driver information system (DIS), multi-function indicator (MFI), or multimedia interface (MMI).

Common symptoms include:

• A text display message stating: "brake booster malfunction"
• An "EPC indicator display" light illuminating
• An "emissions control malfunction display" light appearing
• The "Engine warning lamp is or was on."

These indicators suggest that the brake booster's functionality or operation sequence is incorrect, directly related to the electromechanical brake servo. If you see these warnings, your vehicle may be experiencing the Audi S3 DTC U300100 issue.

Fault Codes (DTCs)

The primary diagnostic trouble code associated with this technical service bulletin is:

Code	Description	Module
U300100	Electromechanical brake servo malfunction	DA0023 (Brake Servo Control)

This code indicates that the electromechanical brake servo, a critical component in your vehicle's braking system, is not operating as expected.

Diagnosis and Trigger Conditions

The DTC U300100 is typically logged when the electromechanical brake servo fails to shut down correctly after the vehicle is parked. This can sometimes be caused by unusual battery voltage conditions. The condition can be either intermittent or static.

• Intermittent DTC: If the complaint cannot be reproduced by a workshop, and the DTC U300100 is logged as intermittent in DA0023, it suggests the issue is not constant.
• Static DTC: If the complaint can be reproduced, and the DTC is logged as static in DA0023, it indicates a persistent problem with the brake servo.

Repair Procedure Summary

The repair procedure for the Audi S3 DTC U300100 varies slightly depending on whether the fault is intermittent or static.

For vehicles with an intermittent DTC U300100:

1. The electromechanical brake servo is designed to shut down correctly during the next bus sleep cycle. A bus sleep cycle occurs when the vehicle's electronic control units power down after a period of inactivity.
2. After this cycle, the intermittent DTC should be able to be deleted using a diagnostic tool.

For vehicles with a static DTC U300100:

1. A technician must work through the ODIS (Offboard Diagnostic Information System) diagnostic system.
2. It is crucial to precisely observe all test plan requirements within ODIS.
3. This process includes performing a bus sleep cycle, during which the electromechanical brake servo will attempt to shut down again.
4. Following a successful bus sleep cycle, the static DTC should switch to an intermittent status.
5. Once intermittent, the DTC can then be deleted, and the engine warning lamp should turn off.

This TSB, identified as 2079459/1, provides the necessary guidance for Audi technicians to address this specific brake servo issue.

Important Warnings

Owners should be aware of the following information regarding this Technical Service Bulletin:

• This TSB is for informational purposes only and is not applicable to any Audi Warranty.
• All part and service references provided in this TSB (2079459/1) are subject to change and/or removal. Always check with your Parts Department and/or ETKA for the latest information and parts bulletins.
• Always check the Repair Manual for fasteners, bolts, nuts, and screws that require replacement during the repair.

What Owners Should Do

If you experience any of the symptoms or warning messages described, particularly the "brake booster malfunction" or an illuminated EPC light, it is recommended to contact an authorized Audi service center. While this TSB is informational and not covered by warranty, a qualified technician can diagnose the Audi S3 DTC U300100 using the specified ODIS procedures and ensure your vehicle's braking system is functioning correctly.

NHTSA ID: 11023826