CAN network

U0121 — Lost Communication with ABS Control Module

Severity: medium Drive with care, to a shop

What U0121 means

U0121 means one control module (or the scan tool) stopped receiving messages from the ABS control module over the CAN bus. Normally the ABS module constantly shares wheel speed and system status with the engine, instrument cluster and other modules. When that link is lost, ABS, traction control and stability control are usually disabled.

Symptoms

  • ABS warning light on
  • Traction or stability control lights on
  • Reduced braking performance under hard braking
  • Warning lights that come and go
  • C-codes (chassis codes) may accompany U0121
  • Several warning lamps on at once
  • Features that depend on the ABS module unavailable

Possible causes

  1. Damaged or corroded ABS module wiring or connector
  2. Faulty ABS control module
  3. Poor power or ground supply to the ABS module
  4. CAN bus open or short circuit
  5. Blown ABS fuse
  6. Low system voltage (weak battery or charging fault)
  7. Damaged wheel speed sensor wiring affecting communication
  8. Another faulty module loading down the bus

How to diagnose

  1. Scan all modules and map which ones fail to communicate
  2. Check battery voltage and charging system output
  3. Check the ABS power fuses and relays
  4. Inspect the ABS connector and harness for corrosion, moisture, chafing and past repairs
  5. Test power and ground at the ABS module connector with a multimeter
  6. Measure CAN bus resistance and voltages at the OBD-II connector
  7. Inspect wheel speed sensor wiring near each wheel
  8. Disconnect suspect modules one at a time, only per a documented procedure, to see if communication returns

Common fixes

  • Replace the blown fuse and restore ABS power and ground connections
  • Repair or replace damaged CAN bus wiring and connectors
  • Charge or replace the battery and fix any charging problem
  • Replace or repair the ABS module (with programming if required)

U0121 on different brands

Ford · manuals
On Ford vehicles such as the Explorer, U0121:00 means a module, for example the PSCM (power steering control module), lost communication with the ABS module; details depend on the controller and model year.

Questions and answers

What does U0121 mean?

It means communication with the ABS control module over the CAN bus was lost. The cause can be power, ground, wiring or the module itself.

Can I drive with U0121?

Yes, carefully, as normal brakes usually work. ABS and stability control may be off though, so leave extra following distance and repair it soon.

What should I check first?

Start with battery voltage, the ABS fuses, then the module connector and grounds, and finally the CAN bus wiring.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.