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Harley Code U0121: Understanding and Troubleshooting the Issue

by

Chase Manhattan

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Owner and Editor at The Wrench Network

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Hey y’all, it’s Chase Manhattan! I’m a life-long automotive and motorcycle enthusiast with a knack for diagnostics that I hope to share with you! Lately, I’ve been running my own motorcycle rental business, which you can read about here, and it has been a great experience that has allowed me to own numerous bikes that I probably couldn’t justify otherwise. Plus, it’s been a great way to experience new places while on vacation. If you haven’t heard of them before, I definitely recommend checking out the Riders-Share platform if you get the itch to ride while away from home, or if you want to explore a cheaper way to own and ride motorcycles!

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If you own a Harley-Davidson motorcycle, you may have encountered the U0121 code. This code indicates a communication problem with the Anti-Lock Brake System (ABS) Control Module. The ABS Control Module sends periodic messages to the Controller Area Network (CAN) bus. If the CAN bus does not receive these messages, the U0121 code is set. The CAN bus uses two wires for communication, CAN high and CAN low.

When the U0121 code appears, it is important to discontinue operation and see a dealer for repair. Continuing to ride with this code can be dangerous, as it indicates a fault in the electrical system. The possible causes of the U0121 code include an open in the CAN high circuit to the ABS module, an open in the CAN low circuit to the ABS module, an open in the ABS system power circuit, or an open in the ABS ground circuit.

Fortunately, there are steps you can take to resolve the U0121 code. First, you should check the wiring and connections to ensure they are secure and free of damage. If the wiring and connections are fine, you may need to replace the ABS Control Module. It is important to address the U0121 code as soon as possible to ensure your safety and the safety of others on the road.

Understanding Code U0121

Definition and Significance

If you own a Harley-Davidson motorcycle, you might have come across the U0121 code at some point. This code indicates that there is a communication problem with the Anti-Lock Brake System (ABS) Control Module. The ABS Control Module sends periodic messages to the Controller Area Network (CAN) bus. If the CAN bus does not receive these messages, the U0121 code is set.

This code is significant because it can affect the safety of your motorcycle. The ABS system is an essential safety feature that prevents your wheels from locking up during hard braking. If the ABS system is not working correctly, it can increase the risk of an accident. Therefore, it is crucial to address the U0121 code as soon as possible.

Common Causes

There are several common causes of the U0121 code. Here are some of the most common causes:

  • Open in CAN high circuit to the ABS module
  • Open in CAN low circuit to the ABS module
  • Open in ABS system power circuit
  • Open in ABS ground circuit
  • Faulty ABS control module
  • Dead battery
  • CAN bus issues

If you are experiencing the U0121 code, it is best to take your motorcycle to a professional mechanic. They will be able to diagnose the issue correctly and take the necessary steps to fix it. Trying to repair the code yourself can be dangerous and can cause further damage to your motorcycle.

In conclusion, the U0121 code is a common issue that Harley-Davidson motorcycle owners face. It is essential to address this code as soon as possible to ensure the safety of your motorcycle. By understanding the common causes of the U0121 code, you can take the necessary steps to prevent it from happening in the future.

Symptoms of the U0121 Code

If you have a Harley Davidson motorcycle and your Malfunction Indicator Lamp (MIL) is on, it could be due to the U0121 code. This code indicates that there is a communication problem with the Anti-Lock Brake System (ABS) Control Module.

Warning Lights on Dashboard

One of the most common symptoms of the U0121 code is the illumination of warning lights on the dashboard. These lights can include the ABS indicator light, the TRAC indicator light, and the ESP/ESC indicator light. If you see any of these lights on your dashboard, it is important to have your motorcycle checked by a professional mechanic as soon as possible.

Vehicle Performance Issues

Another symptom of the U0121 code is vehicle performance issues. You may notice that your motorcycle is not performing as well as it should be. This can include issues with acceleration, braking, and handling. If you experience any of these issues, it is important to have your motorcycle checked by a professional mechanic.

In summary, if you see warning lights on your dashboard or notice any vehicle performance issues, it could be due to the U0121 code. It is important to have your motorcycle checked by a professional mechanic as soon as possible to diagnose and repair the issue.

Diagnosing the Issue

If you’re experiencing the U0121 OBD-II trouble code on your Harley, it’s important to diagnose the issue as soon as possible. Here are the initial steps you can take to diagnose the issue:

Initial Steps

  1. Check the ABS control module and connections to ensure they are properly connected and not damaged.
  2. Check for known ABS issues, such as faulty wheel speed sensors, which can trigger the U0121 code.
  3. Perform a digital multimeter test to check the voltage and resistance of the ABS control module and its connections.

Using OBD-II Scanners

Using an OBD-II scanner can help you diagnose the U0121 code more accurately. Here’s how you can use an OBD-II scanner to diagnose the issue:

  1. Connect the OBD-II scanner to the Harley’s OBD port, which is typically located under the dashboard.
  2. Turn on the ignition and wait for the scanner to establish a connection with the vehicle’s computer.
  3. Use the scanner to perform a scan tool data listening test to determine if there is an ABS exchange of information between modules.
  4. Check for other OBD-II trouble codes that may be present, as they can often provide clues to the underlying issue.
  5. Use the scanner to clear the U0121 code and then test drive the Harley to see if the code returns.

By following these steps, you can diagnose the U0121 code on your Harley and take the necessary steps to fix the issue. Remember to always use caution when working on your vehicle and consult a professional if you’re not comfortable performing the diagnosis yourself.

ABS Control Module

If you’re experiencing issues with your Harley and the U0121 code has been triggered, it’s likely that there is a problem with your Anti-Lock Brake System (ABS) Control Module. In this section, we’ll take a closer look at the role and function of the ABS Control Module, as well as some troubleshooting steps you can take if you’re having issues with this component.

Role and Function

The ABS Control Module is a critical component of your Harley’s anti-lock brake system. Its primary role is to monitor the speed of each wheel and communicate with other systems in your bike to help prevent wheel lock-up during braking. The ABS Control Module uses sensors to detect wheel speed and sends this information to the Engine Control Module (ECM) and other systems in your bike via the Controller Area Network (CAN) bus.

Troubleshooting ABS Module Issues

If you’re experiencing issues with your ABS Control Module, it’s important to troubleshoot the problem as soon as possible to avoid further damage to your Harley. Here are some steps you can take to troubleshoot ABS Module issues:

  1. Check the wiring: One of the most common causes of ABS Module issues is faulty wiring. Check the wiring connections to ensure they are secure and free from damage.

  2. Check the sensors: The ABS Control Module relies on sensors to detect wheel speed. If the sensors are dirty or damaged, they may not function properly. Check the sensors to ensure they are clean and free from damage.

  3. Check for lost communication: The U0121 code indicates that there is lost communication with the ABS Control Module. Check the CAN bus wiring and connections to ensure they are secure and free from damage.

  4. Check the ABS Control Module itself: If all other troubleshooting steps fail, it’s possible that the ABS Control Module itself is faulty. In this case, it may need to be replaced.

Remember, if you’re not comfortable troubleshooting your Harley’s ABS Control Module on your own, it’s always best to seek the help of a professional mechanic. They will have the experience and knowledge necessary to diagnose and repair any issues with your bike’s anti-lock brake system.

Wiring and Electrical Connections

When dealing with the U0121 code on your Harley, it’s important to inspect the wiring harness and connectors for damage or corrosion. Here are some steps you can take to ensure proper wiring and electrical connections:

Inspecting Wiring Harness

First, visually inspect the wiring harness for any signs of damage or wear. Look for any frayed wires, exposed wires, or wires that appear to be pinched or cut. If you notice any damage, you’ll need to replace the damaged portion of the wiring harness.

Next, use a wiring diagram to locate the connectors that are associated with the ABS control module. Inspect these connectors for any signs of damage or corrosion. If you notice any damage, you’ll need to replace the connectors.

Connector and Circuit Checks

Perform a resistance check on the wiring harness and connectors to ensure that there are no open circuits or shorts. You can use a multimeter to perform this check. Make sure to follow the manufacturer’s instructions when using the multimeter.

Check the ground connections for the ABS control module and ensure that they are clean and tight. A loose or corroded ground connection can cause communication issues between the ABS control module and other control modules on the vehicle.

Check the continuity of the wiring harness and connectors to ensure that there are no breaks in the circuit. You can use a continuity tester to perform this check.

Finally, verify the electrical connections between the ABS control module and other control modules on the vehicle. Make sure that all connections are clean and tight.

By following these steps, you can ensure that your Harley’s wiring and electrical connections are in good condition and that the U0121 code is properly diagnosed and resolved.

Advanced Troubleshooting Techniques

If you’re still experiencing U0121 code after trying basic troubleshooting techniques, then it’s time to move on to more advanced techniques. In this section, we’ll discuss two advanced troubleshooting techniques that can help you identify the root cause of the problem.

CAN Bus Signal Analysis

The first technique is CAN bus signal analysis. The CAN bus is responsible for transmitting data between different modules in your Harley Davidson. If there’s a problem with the CAN bus, it can cause the U0121 code to appear. To perform a CAN bus signal analysis, you’ll need a digital multimeter and an oscilloscope.

First, you need to check the CAN high and CAN low wires for continuity. If there’s a break in the wires, it can cause communication problems. Next, you need to check the voltage levels of the CAN high and CAN low wires. The voltage levels should be between 2.5 and 3.5 volts. If the voltage levels are outside of this range, it can indicate a problem with the CAN bus.

Finally, you need to use an oscilloscope to check the waveform of the CAN high and CAN low wires. The waveform should be a square wave with a frequency of 125 kbps. If the waveform is distorted or has a different frequency, it can indicate a problem with the CAN bus.

Reference Voltage and Ground Signal Testing

The second technique is reference voltage and ground signal testing. The U0121 code can also appear if there’s a problem with the reference voltage or ground signal. To perform this test, you’ll need a digital multimeter.

First, you need to check the reference voltage and ground signals of the ABS module. The reference voltage should be between 4.5 and 5.5 volts. If the reference voltage is outside of this range, it can indicate a problem with the ABS module or the wiring. Next, you need to check the ground signal of the ABS module. The ground signal should be less than 0.2 volts. If the ground signal is higher than 0.2 volts, it can indicate a problem with the wiring or the ground connection.

By using these advanced troubleshooting techniques, you can identify the root cause of the U0121 code and fix the problem. However, if you’re still having trouble, it’s best to consult a professional mechanic. They have the expertise and tools to diagnose and fix complex problems.

Repair and Replacement Procedures

If you have encountered the U0121 error code on your Harley, you may need to replace or repair the ABS module or wiring harness. Here are some tips to help you determine when to replace the ABS module and how to repair the wiring harness.

When to Replace the ABS Module

If you have determined that the ABS module is the cause of the U0121 error code, you will need to replace it. Here are some signs that you need to replace the ABS module:

  • The ABS warning light is illuminated on the dashboard.
  • The brake pedal feels spongy or does not respond as it should.
  • The brakes are engaging or disengaging on their own.

Replacing the ABS module is a straightforward process. You will need to remove the old module and install the new one. Be sure to follow the manufacturer’s instructions carefully.

Wiring Repairs

If the U0121 error code is caused by a problem with the wiring harness, you may need to repair or replace it. Here are some tips to help you repair the wiring harness:

  • Inspect the wiring harness for any damage, such as cuts or breaks.
  • Check the wiring harness connectors for any signs of corrosion or damage.
  • Repair any damaged wiring or connectors.

If you are unable to repair the wiring harness, you may need to replace it. Be sure to follow the manufacturer’s instructions carefully.

In conclusion, if you encounter the U0121 error code on your Harley, you may need to replace or repair the ABS module or wiring harness. Be sure to follow the manufacturer’s instructions carefully and seek professional help if necessary.

Preventative Measures and Maintenance

To prevent Harley code U0121 from occurring, it is important to perform regular maintenance on your motorcycle. This includes visual inspections of the electrical system, checking for any wear or burns on wires, and keeping up with any technical service bulletins.

During regular maintenance, it is also important to check the condition of your motorcycle’s ABS system. This can be done by visually inspecting the system for any visible damage or leaks. Additionally, it is important to keep the ABS system clean and free of debris to ensure proper function.

If you suspect that your motorcycle’s ABS system is malfunctioning, it is important to take it to an auto repair shop as soon as possible. The longer you wait to address the issue, the more damage it can cause to your motorcycle and potentially lead to more expensive repairs.

In addition to regular maintenance, it is also important to ride your motorcycle safely and follow all traffic laws. This can help prevent accidents that may cause damage to your motorcycle’s ABS system and trigger Harley code U0121.

By following these preventative measures and performing regular maintenance on your motorcycle, you can help prevent Harley code U0121 from occurring and ensure the safety of yourself and others on the road.

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