**U120E87 Mercedes Fault Code: Diagnosis, Causes, and Solutions**

1. What Does the U120E87 Mercedes Fault Code Mean?

The U120E87 fault code in a Mercedes-Benz signifies a communication issue or a malfunction specifically within the blind spot monitoring (BSM) system. It indicates that the control unit is not receiving the expected signal from one or more of the blind spot sensors. This can result in the system not functioning correctly. Ultimately compromising safety by failing to alert the driver to vehicles in their blind spots.

• Root Cause: The root cause often lies in the blind spot detection radar sensor itself, or the wiring connecting it to the vehicle’s central control unit.

• Component Affected: The primary component affected is the blind spot detection sensor, which is responsible for monitoring the areas beside and slightly behind the vehicle for other vehicles.

• System Impacted: The system directly impacted is the blind spot monitoring system, which is designed to enhance safety by alerting the driver to the presence of vehicles in their blind spots, especially during lane changes.

2. What are the Common Symptoms of the U120E87 Fault Code?

Recognizing the symptoms associated with the U120E87 fault code is crucial for accurate diagnosis and prompt repair. Ignoring these signs can compromise your safety on the road.

• Malfunctioning Blind Spot Monitoring: The most obvious symptom is the failure of the blind spot monitoring system. This includes the warning lights in the side mirrors not illuminating when a vehicle is in the blind spot.
• Warning Lights on the Dashboard: The blind spot assist warning light may illuminate on the dashboard, signaling that the system is not functioning correctly.
• Error Messages in the Vehicle’s Display: The vehicle’s information display might show specific error messages related to the blind spot monitoring system, such as “Blind Spot Assist Inoperative.”
• Intermittent System Operation: The system may work sporadically, functioning correctly at times and failing at others, which can be particularly misleading and dangerous.
• False Alerts: In some cases, the system might generate false alerts, indicating the presence of a vehicle in the blind spot when there is none.

3. What Causes the U120E87 Error Code in Mercedes Vehicles?

Understanding the underlying causes of the U120E87 error code can help you address the problem effectively and prevent future occurrences.

• Faulty Blind Spot Sensor: The most common cause is a defective blind spot sensor. These sensors are typically located in the rear bumper and can fail due to environmental factors or physical damage.
• Wiring Issues: Damaged or corroded wiring connecting the blind spot sensor to the vehicle’s control unit can disrupt the signal and trigger the error code.
• Moisture Intrusion: Water or moisture entering the sensor or its wiring can cause corrosion and malfunction.
• Software Glitches: In rare cases, software glitches in the vehicle’s control unit can lead to communication errors with the blind spot sensor.
• Impact Damage: Minor collisions or impacts to the bumper can damage the sensor or its mounting, leading to the U120E87 error code.

4. How to Diagnose the U120E87 Fault Code in Mercedes?

Diagnosing the U120E87 fault code requires a systematic approach and the right diagnostic tools. CARDIAGTECH.NET offers a range of professional-grade diagnostic equipment to help you pinpoint the exact cause of the problem.

1. Initial Scan: Use an OBD-II scanner to confirm the presence of the U120E87 fault code. This will also reveal any other related fault codes that might provide additional insight.
2. Visual Inspection: Conduct a thorough visual inspection of the blind spot sensors, typically located in the rear bumper. Look for any signs of physical damage, corrosion, or loose connections.
3. Wiring Check: Inspect the wiring and connectors leading to the blind spot sensors. Check for frayed wires, corrosion, and loose connections. Use a multimeter to test the continuity of the wiring.
4. Sensor Testing: Use a diagnostic tool to test the functionality of the blind spot sensors. This may involve checking the sensor’s signal output and response to stimuli.
5. Software Check: Verify that the vehicle’s control unit has the latest software updates. Outdated software can sometimes cause communication issues with the blind spot sensors.

The blind spot detector, often a radar sensor, is crucial for monitoring the areas around your vehicle.

5. Step-by-Step Guide to Fixing the U120E87 Mercedes Fault Code

Follow these steps to address the U120E87 fault code effectively. Depending on the diagnosis, you may need to replace the sensor, repair the wiring, or update the vehicle’s software.

1. Replace Faulty Sensor: If the blind spot sensor is found to be defective, replace it with a new OEM or high-quality aftermarket sensor. Ensure the new sensor is properly mounted and connected.
2. Repair Wiring: If the wiring is damaged or corroded, repair or replace the affected sections. Use proper splicing techniques and weatherproof connectors to ensure a reliable connection.
3. Clean Connections: Clean any corroded connectors with a wire brush and electrical contact cleaner. Apply dielectric grease to protect against future corrosion.
4. Software Update: If a software glitch is suspected, update the vehicle’s control unit to the latest software version. This may require a visit to a Mercedes-Benz dealership or a qualified mechanic.
5. Clear Fault Codes: After completing the repairs, use an OBD-II scanner to clear the U120E87 fault code and any other related codes.
6. Test the System: Test the blind spot monitoring system to ensure it is functioning correctly. Verify that the warning lights illuminate when a vehicle is in the blind spot.

6. What Tools and Parts are Needed to Fix the U120E87 Code?

Having the right tools and parts is essential for a successful repair. CARDIAGTECH.NET offers a wide selection of high-quality tools and parts specifically designed for Mercedes-Benz vehicles.

• OBD-II Scanner: To read and clear fault codes.
• Multimeter: To test the continuity and voltage of the wiring.
• Wire Strippers and Crimpers: To repair or replace damaged wiring.
• Electrical Contact Cleaner: To clean corroded connectors.
• Dielectric Grease: To protect against future corrosion.
• Socket Set and Wrenches: To remove and install the blind spot sensor.
• New Blind Spot Sensor: OEM or high-quality aftermarket replacement.
• Wiring and Connectors: As needed for wiring repairs.

7. Cost of Fixing the U120E87 Error Code on a Mercedes

The cost of fixing the U120E87 error code can vary depending on the cause of the problem and whether you choose to perform the repairs yourself or take your vehicle to a professional mechanic.

• DIY Repair: If you are comfortable performing the repairs yourself, the cost will primarily be the price of the replacement parts. A new blind spot sensor can range from $300 to $700, depending on the brand and quality.
• Professional Repair: If you take your vehicle to a mechanic, you can expect to pay for labor in addition to the cost of parts. Labor costs can range from $100 to $300 per hour, depending on the shop’s rates and the complexity of the repair.

8. Preventing the U120E87 Fault Code in the Future

While some causes of the U120E87 fault code are unavoidable, there are steps you can take to minimize the risk of future occurrences.

• Regular Inspections: Periodically inspect the blind spot sensors and their wiring for any signs of damage or corrosion.
• Protective Measures: Consider applying a protective coating to the sensors and wiring to shield them from environmental factors.
• Careful Driving: Avoid driving in conditions that could expose the sensors to excessive moisture or debris.
• Prompt Repairs: Address any minor damage to the bumper or surrounding areas promptly to prevent further issues.

9. Blind Spot Monitoring System Explained

The Blind Spot Monitoring (BSM) system is a crucial safety feature in modern vehicles, designed to enhance driver awareness and prevent accidents. Understanding how this system works can help you appreciate its importance and maintain it effectively.

• How it Works: The BSM system uses radar sensors, typically located in the rear bumper, to monitor the areas beside and slightly behind the vehicle.
• Sensor Function: These sensors emit radar waves that bounce off objects in the vehicle’s blind spots. The system then analyzes the reflected waves to determine the presence, distance, and speed of other vehicles.
• Alerting the Driver: If the system detects a vehicle in the blind spot, it alerts the driver through visual warnings, such as illuminated icons in the side mirrors, and sometimes audible alerts.
• Operational Conditions: The BSM system is designed to operate under a variety of driving conditions, including daytime and nighttime, and in various weather conditions such as rain and fog.
• System Limitations: It’s important to note that the BSM system has limitations. It may not detect motorcycles, bicycles, or pedestrians, and it may not function correctly in heavy traffic or during sharp turns.

The blind spot holder secures the sensor in place, protecting it from vibrations and impacts.

10. Advanced Diagnostic Techniques for U120E87

For complex cases, advanced diagnostic techniques may be necessary to accurately identify the cause of the U120E87 fault code.

• Oscilloscope Testing: An oscilloscope can be used to analyze the signal waveforms from the blind spot sensors. This can reveal subtle issues that may not be apparent with a standard multimeter.
• Factory Scan Tools: Using a Mercedes-Benz factory scan tool provides access to advanced diagnostic functions and system-specific information.
• Wiring Diagrams: Consulting detailed wiring diagrams can help you trace the circuits and identify any breaks or shorts in the wiring.
• Component Isolation: Isolating individual components of the blind spot monitoring system can help you pinpoint the source of the problem.

11. Choosing the Right Blind Spot Sensor Replacement

Selecting the right replacement blind spot sensor is crucial for ensuring the proper function and reliability of your vehicle’s blind spot monitoring system.

• OEM vs. Aftermarket: OEM (Original Equipment Manufacturer) sensors are made by the same manufacturer that supplied the original part to Mercedes-Benz. Aftermarket sensors are made by third-party manufacturers.
• Quality Considerations: While OEM sensors are typically more expensive, they are often considered to be of higher quality and more reliable. Aftermarket sensors can be a more affordable option, but it’s important to choose a reputable brand.
• Compatibility: Ensure that the replacement sensor is compatible with your vehicle’s make, model, and year.
• Warranty: Check the warranty offered by the manufacturer. A longer warranty can provide peace of mind and protection against defects.
• Reviews and Ratings: Read online reviews and ratings from other customers to get an idea of the sensor’s performance and reliability.

12. Common Mistakes to Avoid When Fixing the U120E87 Code

Avoiding common mistakes can save you time, money, and frustration when addressing the U120E87 fault code.

• Ignoring Related Codes: Pay attention to any other fault codes that may be present, as they can provide valuable clues about the underlying cause of the problem.
• Skipping the Visual Inspection: Always start with a thorough visual inspection of the sensors and wiring before attempting any other repairs.
• Using Inferior Parts: Using low-quality replacement parts can lead to premature failure and further problems down the road.
• Neglecting Wiring Repairs: Don’t overlook the importance of proper wiring repairs. Use high-quality connectors and proper splicing techniques to ensure a reliable connection.
• Failing to Clear Codes: Always clear the fault codes after completing the repairs to ensure that the system is functioning correctly.

13. How Cold Weather Affects Blind Spot Sensors

Cold weather can have a significant impact on the performance and reliability of blind spot sensors.

• Moisture and Ice: Moisture can condense inside the sensor and freeze in cold temperatures, causing it to malfunction.
• Wiring Issues: Cold temperatures can make the wiring brittle and more prone to damage.
• Sensor Sensitivity: Extreme cold can affect the sensitivity of the sensor, making it less accurate.
• Protective Measures: Consider applying a water-repellent coating to the sensors and ensuring that the wiring is properly insulated.

14. Mercedes Blind Spot Assist Legal Information

Understanding the legal aspects of your Mercedes-Benz blind spot assist feature is crucial for safe and responsible driving.

• System Limitations: Be aware that the blind spot assist system is not a substitute for attentive driving. Always check your surroundings before changing lanes.
• Liability: In the event of an accident, you are still responsible for ensuring that it is safe to change lanes, even if the blind spot assist system is not functioning correctly.
• Maintenance: Regularly maintain your blind spot assist system to ensure that it is functioning properly.
• State Laws: Be aware of any state laws that may pertain to the use of blind spot assist systems.

15. Choosing CARDIAGTECH.NET for Your Mercedes Repair Needs

When it comes to repairing your Mercedes-Benz, choosing the right supplier for diagnostic tools and replacement parts is essential. CARDIAGTECH.NET offers a comprehensive range of products and services to meet your needs.

• Wide Selection: We offer a wide selection of high-quality diagnostic tools and replacement parts specifically designed for Mercedes-Benz vehicles.
• Expert Advice: Our team of experienced technicians can provide expert advice and guidance to help you diagnose and repair your vehicle.
• Competitive Prices: We offer competitive prices on all of our products and services.
• Customer Satisfaction: We are committed to providing excellent customer service and ensuring your satisfaction.
• Convenient Ordering: You can easily order our products online or by phone.

Table: Blind Spot Sensor Specifications

Table: Comparison of Blind Spot Sensor Brands

Table: Common U120E87 Troubleshooting Steps

16. Integrating U120E87 Fixes with Routine Maintenance

Incorporating U120E87 fixes into your routine maintenance schedule is a proactive way to ensure the longevity and reliability of your Mercedes-Benz.

• Regular Check-ups: Schedule regular check-ups with a trusted mechanic to inspect the blind spot monitoring system.
• Sensor Cleaning: Keep the blind spot sensors clean and free from debris.
• Wiring Maintenance: Inspect the wiring for wear and tear during routine maintenance.
• Software Updates: Ensure that your vehicle’s software is up to date.

17. Understanding Mercedes Diagnostic Trouble Codes (DTCs)

Understanding Mercedes Diagnostic Trouble Codes (DTCs) is vital for anyone looking to maintain or repair their vehicle effectively. DTCs are codes that the vehicle’s onboard computer generates when it detects a problem.

• OBD-II System: Mercedes-Benz, like all modern vehicles, uses the OBD-II (On-Board Diagnostics II) system, which provides standardized diagnostic codes.
• Code Structure: DTCs consist of a five-character alphanumeric code. The first character indicates the system (e.g., P for Powertrain, B for Body, C for Chassis, and U for Network). The second character indicates whether the code is generic (0) or manufacturer-specific (1). The third character specifies the subsystem, and the last two characters indicate the specific fault.
• Reading DTCs: DTCs can be read using an OBD-II scanner. This tool connects to the vehicle’s diagnostic port and retrieves the stored codes.
• Clearing DTCs: After addressing the issue, it’s important to clear the DTCs using the scanner. This resets the system and turns off the warning lights.
• Importance of Accurate Diagnosis: Correctly interpreting DTCs is crucial for accurate diagnosis and effective repair.

18. The Role of Radar Technology in Blind Spot Monitoring

Radar technology plays a pivotal role in the functionality of blind spot monitoring systems. These systems rely on radar to detect and track vehicles in the driver’s blind spots.

• How Radar Works: Radar (Radio Detection and Ranging) technology uses radio waves to determine the distance, speed, and direction of objects.
• Radar Sensors: Blind spot monitoring systems use radar sensors that emit radio waves. When these waves encounter an object, such as another vehicle, they bounce back to the sensor.
• Signal Analysis: The system analyzes the reflected waves to determine the presence, distance, and speed of the object.
• Advantages of Radar: Radar technology offers several advantages, including the ability to function in various weather conditions, such as rain, fog, and snow. It also provides accurate and reliable detection.
• Sensor Placement: Radar sensors are typically located in the rear bumper of the vehicle, strategically positioned to cover the blind spot areas.

19. How to Use a Multimeter for U120E87 Diagnosis

A multimeter is an essential tool for diagnosing electrical issues, including those related to the U120E87 fault code. It allows you to test the continuity, voltage, and resistance of the wiring and components.

1. Continuity Testing: To check the continuity of a wire, set the multimeter to the continuity setting (usually indicated by a sound wave symbol). Touch the probes to each end of the wire. If the multimeter beeps or shows a low resistance reading, the wire is intact.
2. Voltage Testing: To test the voltage, set the multimeter to the voltage setting (DC voltage for most automotive applications). Connect the black probe to a ground point on the vehicle and the red probe to the wire you want to test. The multimeter will display the voltage reading.
3. Resistance Testing: To test the resistance of a component, set the multimeter to the resistance setting (Ohm symbol). Disconnect the component from the circuit and touch the probes to each end of the component. The multimeter will display the resistance reading.
4. Safety Precautions: Always disconnect the battery before performing any electrical tests to avoid the risk of electric shock or damage to the vehicle’s electrical system.

20. Benefits of Updating Your Mercedes Software

Keeping your Mercedes-Benz software up to date is crucial for optimal performance, security, and compatibility with the latest features and technologies.

• Improved Performance: Software updates can improve the performance of various systems, including the engine, transmission, and braking system.
• Bug Fixes: Updates often include bug fixes that address known issues and improve the stability of the systems.
• New Features: Software updates may introduce new features and functionalities to enhance the driving experience.
• Security Enhancements: Updates can also include security enhancements to protect against cyber threats and vulnerabilities.
• Compatibility: Keeping your software up to date ensures compatibility with the latest devices and technologies.

21. Impact of Aftermarket Accessories on Blind Spot Monitoring

Adding aftermarket accessories to your Mercedes-Benz can sometimes interfere with the proper functioning of the blind spot monitoring system.

• Sensor Interference: Some aftermarket accessories, such as certain types of bumper stickers or modifications, can interfere with the radar signals emitted by the blind spot sensors.
• Electrical Issues: Improperly installed electrical accessories can cause electrical interference or damage the vehicle’s wiring, leading to issues with the blind spot monitoring system.
• Compatibility: Ensure that any aftermarket accessories you install are compatible with your vehicle’s systems and do not interfere with the functioning of the blind spot monitoring system.
• Professional Installation: Consider having aftermarket accessories installed by a professional to ensure that they are properly installed and do not cause any issues.

22. U120E87 and Mercedes Safety System Integration

The U120E87 fault code and the blind spot monitoring system are closely integrated with other safety systems in your Mercedes-Benz, forming a comprehensive safety network.

• Collision Prevention Assist: The blind spot monitoring system works in conjunction with the collision prevention assist system to help prevent accidents.
• Lane Keeping Assist: The lane keeping assist system uses the same sensors as the blind spot monitoring system to detect lane markings and prevent unintentional lane departures.
• Adaptive Cruise Control: The adaptive cruise control system uses radar technology to maintain a safe distance from the vehicle ahead.
• Pre-Safe System: The pre-safe system prepares the vehicle for a potential collision by tightening the seatbelts, adjusting the seats, and closing the windows.
• Importance of System Integration: The integration of these safety systems enhances overall safety and provides a more comprehensive level of protection.

23. Seeking Professional Help for U120E87

While many repairs can be performed by DIY enthusiasts, certain situations warrant seeking professional help for the U120E87 fault code.

• Complex Issues: If you are unable to diagnose the problem or if the repairs are complex, it’s best to consult a qualified mechanic.
• Specialized Tools: Some repairs require specialized tools or equipment that are not readily available to the average DIYer.
• Safety Concerns: If you are not comfortable working on your vehicle’s electrical system or if you have any safety concerns, seek professional help.
• Warranty Considerations: Performing repairs yourself may void the vehicle’s warranty, so it’s important to consider this before attempting any repairs.

24. Future Trends in Blind Spot Monitoring Technology

The technology behind blind spot monitoring systems is constantly evolving, with new advancements and features being introduced regularly.

• Improved Sensors: Future systems will likely use more advanced sensors that offer greater accuracy and range.
• Artificial Intelligence: AI algorithms will be used to analyze sensor data and provide more intelligent alerts.
• Integration with Autonomous Driving: Blind spot monitoring systems will play a key role in the development of autonomous driving technology.
• Enhanced User Interface: Future systems will feature more intuitive and user-friendly interfaces.
• Increased Safety: The ultimate goal of these advancements is to improve safety and reduce accidents.