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U0121 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


U0121 is an OBD-II Code that refers to Lost Communication With Anti-Lock Brake System (ABS) Control Module


A U0121 code is a generic communication systems diagnostic trouble code that refers to the communications circuit between the Anti-Lock Brake (ABS) control module and other control modules on the vehicle.

This communications circuit is most commonly referred to as Controller Area Network bus communications, or simply put, CAN bus. Without this CAN bus, control modules cannot exchange information, and your scan tool may not be able to communicate with the vehicle, depending on which circuit is affected.


Common causes for this code include:

  • Bad Anti-Lock Brake System (ABS) Control Module
  • Anti-Lock Brake System Control Module circuit poor electrical connection
  • Anti-Lock Brake System Control Module harness is open or shorted


The symptoms of a U0121 code are:

  • Malfunction Indicator Lamp (MIL) on 
  • ABS indicator light on 
  • TRAC indicator light on (depending on manufacturer) 
  • ESP/ESC indicator light on (depending on manufacturer)


To diagnose a U0121 DTC code, a technician would:

  1. Check the Anti-Lock Brake (ABS) Control Module and connections
  2. If necessary, check for known ABS issues (faulty wheel speed sensor(s), etc.)
  3. Perform a PCM Scan Tool Data Listening Test to determine if there is an ABS exchange of information between modules. If no data is present, the ABS control module and related circuits would be checked and replaced if necessary. 
  4. If data is present, further diagnosis may be needed to identify which control module circuit is shorted or open.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a U0121 code:

  • Not checking the freeze frame data to determine the conditions under which the DTC was set.
  • Not checking the documents for the vehicle to ensure that the code is correct and not a different code.
  • Using diagnosis equipment that does not properly interpret the DTCs and advise them of their scan results.
  • Not performing all tests or swap tests necessary to determine what components are faulty. It may be easy to identify one or two components that are faulty but to really ensure the diagnosis, every test should be done.
  • Before replacing any components or parts, always check that they are compatible with the vehicle.

How serious is this?

A U0121 code is considered to be serious. If a vehicle is exhibiting symptoms, it should be diagnosed as soon as possible to avoid further damage and the possibility of an accident.

What repairs can fix the code?

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The following are solutions that may fix this problem:

  • First, record all fault codes that may be present.
  • Check the location of the wiring and ABS Brake Control Module using the service manual. Examine for obvious signs of damage, such as wear, corrosion, or burns in the wiring. Repair any damaged areas.
  • Check the resistance, reference voltage, continuity, and ground signal of the system wiring harness according on the service manual’s instructions. If you detect an out-of-range value, take the appropriate corrective measures.

Related codes

A U0121 is related to and may be accompanied by the following codes:

P0021, P0117, P0220, P0732, P0457, P0332, U0401, P2005, P0358, P0033, P0868, P0735


In conclusion, the U0121 code is a communication error between the ABS Control Module and other control modules on the vehicle. It may be caused by a faulty ABS Control Module, wiring harness, or related circuits. Check for obvious damaged components and repair as necessary.