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OBD-II · Network and communication (U)

U0121

Lost communication with ABS control module

What it means

Another module has stopped receiving messages from the ABS/ESP brake control unit.

Symptoms

  • ABS, ESP/traction and brake warning lights on
  • Speedometer may stop working
  • Gearbox or cruise control may misbehave because wheel speed data is missing

Likely causes, most likely first

Likely causes, most likely first
No.Cause
01weak 12V battery or poor earth
02blown ABS fuse (often a high-current fuse in the engine bay fuse box)
03corroded ABS module connector (the unit sits low in the engine bay and gets wet and salty)
04internal failure of the ABS module electronics
05damaged CAN wiring to the ABS unit

What to check, step by step

  1. 01Stop safely and arrange recovery if braking changes, the red brake warning remains on with the parking brake released, or the vehicle instructs you to stop; with only an ABS warning, follow the handbook and arrange prompt diagnosis
  2. 02Check 12V battery and earths
  3. 03Check both ABS fuses (pump motor and module supply)
  4. 04Check whether the ABS module answers a scan tool
  5. 05Unplug and inspect the ABS module connector for corrosion and green deposits
  6. 06Check power, ground and CAN at the ABS connector before replacing or repairing the module