U0121 on Audi: How to Diagnose & Fix
Lost Communication with ABS Module
1 Audi model · 1 documented issue
U0121 on Audi by model: symptoms, diagnosis and fix
Audi TSB 2037761/1 covers a specific situation in which a 2012-2015 A6 shows ABS/ESP warnings and the engine does not switch off with a normal press of START ENGINE STOP. The ignition system uses the ABS control unit's vehicle-speed signal to determine whether the car is stationary. A control-module/signal fault can remove that signal; prolonged wheel slip in winter can also make wheel speeds appear implausible and temporarily deactivate ABS even when there is no permanent system error. The previous record duplicated the older C5 Bosch-module card and sent users to a 4B0614517P search without establishing vehicle generation, DTC, system version, or a failed module.
Common Symptoms
- ABS or ESP warning is shown in the instrument cluster
- A normal press of START ENGINE STOP does not switch off the engine while stopped
- The condition may follow loss of the ABS speed signal or prolonged winter wheel slip
How to Fix
Bring the vehicle to a complete stop in a safe location. Audi states that below 6 mph (10 km/h), the emergency-off function is activated by holding START ENGINE STOP while pressing the brake pedal. Diagnose the vehicle according to the stored DTCs rather than ordering an ABS module from the warning alone. If the warning followed sustained winter wheel slip, Audi notes that the system may become fully active again after the ignition is switched off and the vehicle is restarted; persistent warnings still require diagnosis. Do not use the emergency-off procedure while driving except as described by the owner's manual and Audi safety information. This informational bulletin specifies no replacement part, so this record has no commerce link.
Owner tips & cautions
- WarningStop safely before using the emergency-off function. At speeds below 6 mph, Audi specifies holding START ENGINE STOP while pressing the brake pedal.
- TipSave the full DTC scan and the conditions that preceded the warning, including sustained wheel slip. The bulletin directs diagnosis by DTC and does not identify a universal ABS replacement part.
Sources for this issue (1)
U0121 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does U0121 mean on a Audi?
U0121 stands for "Lost Communication with ABS Module." U0121 is a network communication code meaning a control module on the vehicle's data bus has lost communication with the ABS (anti-lock brake system) module. Modern cars share information over a CAN bus, and when other modules stop receiving expected messages from the ABS unit, this code sets. Because the ABS module also handles traction control and stability control on most vehicles, you may see multiple warning lights and lost features. It points to a communication or power problem with the ABS module rather than a worn brake part. On Audi it is documented across 1 model: A6.
What are the most common causes of U0121?
The most common causes of U0121 are: Loss of power or ground to the ABS module, Blown fuse feeding the ABS controller, Damaged, corroded, or loose ABS module connector, Open, shorted, or chafed CAN bus wiring, Internal failure of the ABS control module, Corrosion or water intrusion in connectors/harness, Faulty gateway module disrupting bus communication. The specific cause varies by vehicle.
How much does it cost to fix U0121 on a Audi?
Repair costs vary widely with the root cause and Audi model; each issue on this page carries its own estimate where one is published.
Looking for U0121 on a different make?
View U0121 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.