2025-2026 Audi MIL On DTC U041800 TSB 2078871/2

Symptoms

Owners of affected 2025-2026 Audi models may observe the "engine warning lamp" (MIL) illuminated on their instrument cluster. This light indicates that the vehicle's onboard diagnostic system has detected a fault. While the lamp is on, there might not be any immediate noticeable driving symptoms. It is important to note that the complaint of the illuminated lamp may not always be reproducible in the workshop, as the lamp can sometimes turn off after a bus sleep cycle. The presence of the Audi MIL on DTC U041800 is the primary symptom.

Fault Codes / DTCs

When the engine warning lamp is illuminated, a diagnostic scan will typically reveal the following fault code:

Code	Description	Module	Symptom Code	Diagnostic Address
U041800	ABS Control Module, implausible signal	ABS Control Module	487509	812C

This specific diagnostic trouble code (DTC) indicates an issue with the Anti-lock Braking System (ABS) control unit's communication or signal integrity. The symptom code 487509 further specifies the nature of the implausible signal.

Technical Background and Cause

Audi Technical Service Bulletin 2078871/2 explains that the root cause of the Audi MIL on DTC U041800 is a delay in signals provided by the ABS control unit. This delay can inadvertently trigger the engine warning lamp. This condition is particularly prone to occurring if the vehicle's electrical system voltage drops below 12V. Essentially, a temporary dip in voltage can cause the ABS module to send a delayed or "implausible" signal, leading to the DTC being logged and the warning lamp illuminating.

Production Solution

Audi has indicated that there is currently "Not applicable" as a production solution for this specific issue. This implies that the vehicle design or manufacturing process has not yet been updated to prevent this condition from occurring.

Diagnosis and Repair Procedure Summary

For technicians addressing the Audi MIL on DTC U041800, TSB 2078871/2 provides specific guidance. The service information aims to support the repair process, which involves a straightforward diagnostic and clearing procedure.

1. Ensure 12V Power Supply: Before initiating any diagnostic or repair steps, technicians must ensure that the vehicle has a stable 12V power supply. This is crucial because low voltage is identified as a trigger for the implausible signal.
2. Delete the DTC: Once a stable power supply is confirmed, the stored DTC U041800 should be deleted from the vehicle's system using appropriate diagnostic tools.
3. Re-evaluate the Vehicle: After deleting the DTC, the vehicle should be re-evaluated based on the customer's clear complaints. It is important to remember that the original customer complaint may have already been resolved by a bus sleep cycle, which can clear the warning lamp even if the DTC remains stored.

Audi also notes that "The measures on how to correctly provide the signals and revise the warning messages are in development." This indicates that while a temporary resolution is provided, a more permanent software or system update is being worked on by the manufacturer.

Important Warnings for Owners

Owners should be aware of a critical detail regarding this service bulletin. The TSB explicitly states that "Billing to this TSB number is not permitted." This means that any diagnostic or repair work performed under the scope of TSB 2078871/2 should be covered under warranty and should not result in charges to the customer. This TSB is intended to provide support during warranty work.

Source: NHTSA TSB 2078871/2