Trouble code U0121 Fix: Problem with ABS Control Module

Having a car is like having a pet. It would help if you don’t want any trouble using your vehicle, then you need to use the car carefully. And as a savior, you must know about board diagnostics or OBD2 scanners to find the problem quickly. Just like that, sometimes, you see the U0121 code on the screen of your car. You need to know the reason behind this problem and how to solve this.

What is the meaning of the trouble code U0121?

The OBD2 scanner, U, is mainly used for user network problems. This trouble code U0121 is especially meaning of the communication problem of the anti-lock brake system or ABS control module. That means losing the communication with the ABS control module with another module or Power train module(PCM). In some applications, this code checks the engine light after a problem with communication of 0.25 seconds with other modules.What Does the Engine Code U0121 Mean

So a trouble code U0121 is a generic communication system. This code refers to the connection between the Anti-lock brake(ABS) control module and other vehicle modules. This communication circuit is most used as a controlled area bus network or CAN bus. Because without this CAN bus, no control bus can exchange the information between one another, and you also miss to see the code on your car screen. And your scanner is also unable to find the problem with all communication systems. It also depends on how much the circuit is damaged.

What are the symptoms of the code U0121?

If you want to solve this problem quickly, you need to notice some symptoms in your car. Like:

  1. MIL or Malfunction Indicator Lamp is one,
  2. The motor doesn’t start properly.
  3. Limited Acceleration,
  4. Safe mode failure to act.
  5. ABS light is on.

What is the reason behind this Code U0121?

 There is also some reason for this trouble code U0121. Like:

  1. Malfunction in Anti-Lock brake or ABS control module,
  2. The poor condition of the Battery,
  3. The damaged or broken electrical connection of the ABS control module,
  4. Open or short harness system in Anti-Lock brake or ABS control module.
  5. If accidentally replaced the control system that is not related to ABS. Sometimes, it loses communication.

How does Diagnosis trouble with the U0121 code?

Diagnosis of the problem is one of the critical parts of this code. But if you notice carefully, you can quickly point out the problem:

  1. Check the broken connection or cables of the internal module,
  2.  Check whether the ABS control module has any defects or not.
  3. If any recent replacement is done in the control module, check it can easily communicate with all the control modules.
  4. Check whether the Battery is okay or not, because sometimes a battery’s poor performance also gives the low voltage of the problem.

Some mistakes you face when Diagnosis the problem:

You may have often made some common mistakes to find out the problem. Like:

  1. Forget to check the freeze frame data, which determines the condition and depends on whether the DTC was set.
  2. Forget to check the vehicle’s document to know if the code is correct, and also, this is the exact code for the system.
  3. Sometimes the diagnostic equipment could not scan the problem properly from the DTCs.
  4. Always check the compatibility with the car; check the replacement equipment of the parts.
  5. Forget to run the performing test to determine the problem with any component.

How to fix the problem with the code U0121?

 After you know every detail of the trouble code U0121, then you must want to know how to solve this problem. You need to follow some simple steps to solve this problem. Like:lost communication with brake system control module

  1. First, go through all the fault codes present in the vehicle and try to solve the problem.
  2. Then check the Anti-Lock brake or ABS total wiring system; you also use the control module manual book to find out the problem. Now inspect every sign of damage like corrosion, wear, or burns in the wiring time. After finding out the problem, repair or replace the damaged areas.
  3. Now take the help from the manual for guidelines, test the resistance, and reference the harness wiring system’s voltage, continuity, and ground signal. If you detect any problem there, take a reasonable step to solve the problem.
  4. Some updated vehicles like Mazda cars have a code reader or scanner connected with engine operation. For this type of car, after solving the problem, clear the fault code from the scanner with the help of the manufacturer’s instruction.

Solving the problem is also not so complicated. Just follow this step to repair the problem. You may think how important it is to crack this code. It’s pretty much essential to fix the issue as soon as possible. Because if you see a problem in the vehicle, it may lead you to damage and any accident while using the car. This problem is associated with the break. So I always suggest you solve the problem when you notice the code U0121.


At last, you know the reason behind this error code U0121. This problem is connected with the Anti-lock brake or ABS control module with the other control module in the car. This is also the reason behind the fault in the ABS control module or damaged wiring harness and circuit, or responsibility for the battery performance. And this problem may lead you to an accident in the future. So try to repair this problem to avoid any issues.

Kevin Nicholas is an automotive technician who is a genius at software and hardware-related issues. He manually tested more than a hundred OBD scanners and gave his honest opinion on whether the device was worth the money or not. His in-depth OBD review articles help people choose the right product, whether it is a European, American, or Asian vehicle. He completed his Automotive Specialized Training Course at Universal Technical Institute and has more than 15 years of experience in the field.

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