Code Definition

U0121 — Lost Communication with Anti-lock Brake System Module

System: Network Communication  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About U0121

U0121 is a generic (obd-ii standard) network communication diagnostic trouble code. It belongs to the U-prefix family of codes, which covers inter-module network communication (CAN, LIN, FlexRay, or similar) faults.

Scanning & clearing: Network (U) codes report communication faults between modules. When diagnosing U-codes, checking the vehicle's CAN bus integrity, supply/ground at each module, and reading all modules for related faults is usually the first step.

Because U0121 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The Antilock Brake System (ABS) module, also referred to as the Electronic Brake Control Module, broadcasts wheel speed, vehicle speed, brake pressure, yaw rate and lateral acceleration data onto the CAN data bus, where the ECM, TCM, instrument cluster, and steering control modules all rely on it. DTC U0121 is stored by a receiving module when the periodic messages from the ABS module stop arriving, either failing to appear within a defined window after ignition on or dropping out while the vehicle is being driven. The ABS module is bolted directly to the hydraulic modulator in the engine bay,...

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