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U0121 Network

Lost Communication with ABS Control Module

The CAN messages from the ABS module are missing. ABS, ESP/ESC and traction control all become inactive (warning lamps illuminate), the speedometer may go to zero on cars that derive vehicle speed from the ABS module, and adaptive cruise control is disabled.

Affected components

  • Abs control module
  • Can bus wiring

Common causes

  • High likelihood: ABS module unpowered (often a corroded ground or fuse)
  • Medium likelihood: CAN wiring to ABS broken
  • Medium likelihood: ABS module failed

Possible symptoms

  • Multiple warning lights may illuminate
  • Affected modules may go into limp mode or shut down
  • Specific functions inoperative depending on module
  • Diagnostic tools may show communication faults

Diagnostic notes

  • The malfunction indicator lamp may illuminate

Sources

Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.

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