U0121 – Data bus: anti-lock brake system (ABS) control module – no communication

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By Reinier (Contact Me)
Last Updated 2016-06-30
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
U0121 Data bus: anti-lock brake system (ABS) control module - no communication
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Table of Contents

  1. What Does Code U0121 Mean?
  2. What are the common causes of code U0121?
  3. What are the symptoms of code U0121?
  4. How do you troubleshoot code U0121?
  5. Codes Related to U0121
  6. Get Help with U0121

What Does Code U0121 Mean?

OBD II fault code is defined as “Data bus: anti-lock brake system (ABS) control module – no communication”, and is set when communication between the ABS brake system control module and the PCM (Powertrain Control Module) is lost for a specified time. On most applications, the code will be set, and the CHECK ENGINE light illuminated after an interruption in communication of 250 milliseconds (0.25 sec).

During normal vehicle operation, all control modules are connected to a system known as the General Motor Local Area Network (GMLAN). In essence, this system monitors output data from all control modules involved in the operation of the vehicle, and it can be likened to a computer network that monitors operational systems in a complex, automated manufacturing environment where several controllers work together to deliver a final product. In this case, the final product is the safe and efficient operation of the vehicle.

In practical terms, the GMLAN system is a sort of clearing house for all information exchanged between the various modules that control systems like the ABS brakes (and associated safety systems like Traction and Stability Control), the transmission, fuel management, and others. Moreover, the GMLAN system supervises communication between the various control modules, each of which contains prerecorded information about the type(s) of signals that need to be exchanged between them to ensure efficient and trouble free operation of the vehicle.

In addition, the PCM expects to receive regular availability, or status, reports from each control module in the system. In the case of the ABS control module, an interruption of more than 250 milliseconds will set the code, since an interruption in communication can affect associated systems like Traction and Stability Control, since these safety systems share some of the circuitry (and functionalities) of the ABS system.

To facilitate fault diagnosis, each availability report contains a unique code that identifies the control module that sends the report, thus enabling the PCM to identify defective modules correctly.

The image below shows a typical ABS actuator with the control module contained in the black plastic box. Note the pins in the connector; on this particular unit the ABS brakes, Electronic Traction Control, and Electronic Stability Control share the control module, hence the large number of pins in the connector. Note that this example is only representative of ABS actuators, and that designs, locations, and functionalities differ widely between applications.


What are the common causes of code U0121?

The possible causes of the failure of the ABS control module (or any other controller for that matter) are many and varied, including some causes not related to the ABS circuit at all. Below are some possible causes of code U0121-

  • Damaged, burnt, shorted, corroded, or otherwise damaged wiring and connectors.


  • Defective ABS control module.
  • In some cases, replacement of other, unrelated control modules such as transmission control modules can lead to a loss of communication, and hence the setting of code U0121.

  • Voltage spikes due to incorrect jump starting procedures or other causes can damage one or more controllers, including the ABS control module.

  • On some vehicles, most notably many Mazda products, this code can sometimes be set if a scanner or code reader is connected while the engine is running or the ignition is switched on. However, in most of these cases, the presence of the code is merely a “glitch” that does not indicate an actual problem, and simply clearing the code by following the prescribed procedure will resolve the glitch.

  • Low battery voltages or defective batteries can cause sporadic faults throughout the electrical system, and it can even damage some controllers, including the ABS control module. Note that where low battery voltages or defective batteries are involved, other codes will usually be present along with U0121.

  • Failed, or failing PCM. Note that this is a rare event, and it is far more likely that the ABS control module will fail before the PCM does.

What are the symptoms of code U0121?

On some applications there may be no outward signs of trouble apart from the presence of code U0121, and an illuminated warning light. However, some applications can display severe symptoms, but note that not all symptoms will be present on all applications all of the time. Moreover, the severity of some symptoms may differ between applications. Some common symptoms of code U0121 could include the following-

  • Vehicle could enter limp mode as a precautionary measure to reduce speed. The extent of limp mode could also vary- in some cases, throttle control by the driver may be limited and/or the transmission could be prevented from shifting.
  • In some cases, the vehicle may not start.
  • In some cases the instrument cluster fails, but when this happens other codes may be present, most often code U0155.

How do you troubleshoot code U0121?

WARNING: Apart from a wiring diagram, a good quality digital multimeter, and sometimes an oscilloscope, diagnosing code U0121 requires specialist knowledge of the application being worked on, as well as above average knowledge of automotive electronics and diagnostic procedures. Diagnosing this code is therefore not recommended for persons who do not possess these skills.

NOTE #1: While most repair manuals will assist in locating the various control modules, they will rarely contain information on how to test a particular module. Therefore, unless the manual clearly describes testing procedures for a particular control module, do NOT attempt to open or repair a suspected defective module. Doing so could cause serious damage to the vehicle’s electrical system or worse, cause a crash when the “repaired” module fails unexpectedly.

NOTE #2: In most cases, the repair procedure for code U0121 is make and model specific, since the replacement module sometimes needs to be integrated with, or matched to the PCM. While it is sometimes possible to do this on a DIY basis, it is recommended that this step be performed by a dealer or specialist repair shop that has the required equipment to verify that the integration is successful.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information could be useful should an intermittent fault be diagnosed later on.

Step 2

Consult the manual to locate the wiring harness that serves the ABS brake system, as well as to locate the ABS brake control module. Note however that this harness might be incorporated into the main wiring harness, so use extreme care when removing insulation to expose the relevant wiring to avoid damaging unrelated wiring.

Step 3

Once the relevant wiring is exposed and the control module located, perform a thorough visual inspection of the wiring and connectors. Look for wiring and connectors that are burnt, chafed, shorted, corroded, or otherwise damaged.

NOTE: If damaged wiring and/or connectors is found, resist the temptation to make repairs to the damaged wiring. The better option is always to replace the relevant harness to avoid resistance and/or continuity issues with poorly executed repairs.

Step 4

After replacing any wiring, clear the code, and operate the vehicle to see if the code returns. If the code does return, it is likely that the control module itself is defective; proceed as follows-

  • Consult the manual on the correct procedure to test for resistance, ground, reference voltage and continuity on all associated wiring. Note that the ABS control module needs to be isolated from the main electrical system for resistance checks to be accurate. Where a manual does not describe testing procedures, it is safe to assume that the control module cannot be tested reliably. In these cases, replacing the control module is the only option.

  • If the module has to be replaced, consult the manual on the integration process (if applicable) before the old module is removed. In some cases, the vehicle may become inoperable until the new module is integrated, so if you cannot perform the procedure yourself, you need to be able to drive the vehicle to a dealer to have this step performed.

  • If the manual describes the replacement and integration process, be sure to follow the directions exactly to prevent damage to the electrical system. Clear all codes after replacing the module with an OEM part, and operate the vehicle to see if the code returns. If the code does return, it is likely that the integration process was not successful. Consult the manual on the steps required to repeat the process, but be sure to clear all codes before testing the vehicle again.

NOTE: Not all ABS control modules require integration, but consult the manual to be sure that it is not required for the application being worked on. However, where it is required, be sure to follow the directions in the manual exactly, or the process might fail.

Step 5

If there is no damage to wiring and connectors, and all electrical values fall within the manufacturer’s specification, replacement of the ABS control module will almost always resolve the problem, but to be sure that it has been resolved definitively, complete a few drive cycles before scanning the system again. Bear in mind that on most applications a single failure will set code U0121, so if it does not return after a few cycles, it is safe to assume that the problem is resolved.

However, if the code does return after a few cycles, it is likely that there is an intermittent fault present. This type of fault can be extremely difficult to trace, especially in a complex electronic circuit such as the ABS system, and it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

NOTE: On some applications it is sometimes necessary to expose the wiring in large parts of the main harness. In these cases, it is important to tape up the wiring to the same standards as the original insulation to prevent damage through chafing, rubbing, or contact with hot surfaces. Also be sure to secure the harness by refitting all retaining clips, brackets, and other fasteners.

Note that while code U0121 generally refers to a communication problem between the ABS brake control module and the PCM, not all manufacturers have assigned this problem to this code. In some cases, code U0121 is an engine code that refers specifically to a communication issue in the variable valve timing mechanism control circuit. It is recommended that whenever code U0121 is present on any vehicle, the exact definition be researched in how it relates to the application being worked on.

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