U0121 on Audi
Lost Communication with ABS Module
U0121 on Audi vehicles indicates lost communication with abs module. Au7o has documented this code across 1 Audi model — most commonly on A6. 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.
Common Causes of U0121
- •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
U0121 on Audi by Model
Audi A6(1 issue)
- 2012-2015 A6 ABS/ESP Warning With Engine Will Not Switch Off2012-2015
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.
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View U0121 across all makes →Frequently Asked Questions
What does U0121 mean on 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 specifically, this code is documented across 1 model.
What causes U0121 on Audi vehicles?▼
Common causes on Audi: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix U0121 on a Audi?▼
Repair costs vary widely depending on the root cause and specific Audi model.
Which Audi models have U0121 documented?▼
Au7o has documented U0121 on 1 Audi model: A6.