Solved Collision Prevention Assist Plus Inoperative Warning

Mercedes “Collision Prevention Assist Plus Inoperative” — What It Means

Collision Prevention Assist Plus Inoperative is a warning that can catch any driver off guard. This advanced safety system is designed to help prevent rear-end collisions by issuing visual and audible alerts and, if necessary, initiating braking.

When it becomes inoperative, the loss of this added layer of protection can be unsettling, especially for those who depend on it for daily driving confidence.

Collision Prevention Assist Plus Inoperative was the key issue in two real-world case studies featured in this article. We walk you through how technicians identified the root causes and resolved them using practical, step-by-step methods.

Whether you’re a car owner seeing this warning or a technician looking for guidance, these examples provide clear, actionable solutions to get the system working again.

Common Causes

– Low or unstable voltage (main or auxiliary battery): Insufficient power can shut off radar operation

– Dirty or blocked forward radar sensor: Debris disrupts radar readings.

– Blown fuse or wiring issue: A fuse failure or loose connector in the radar/module circuit.

– Bad sensor/module or CAN/FlexRay communication fault: Hardware failure or network interruption

– Software glitch or misalignment: Modules sometimes need resets, calibrations, or updates.

How to Diagnose & Fix

– Replace faulty radar or control unit: If moisture, physical damage, or module errors persist, professional replacement may be needed.

– Restart & charge the car: Park, turn off ignition for 2–5 minutes, then restart. Driving for ~30 minutes can reboot systems and resolve voltage-related inoperability

– Check the battery: Inspect both main and auxiliary batteries. Weak batteries (under ~12.4 V off, or low auxiliary ~0.5 V) are common culprits.

– Clean the radar sensor: Located behind the front bumper emblems or grille. Remove dirt, bugs, or ice and restart.

– Inspect fuses and wiring: Replace blown fuses; check connectors for corrosion or makeshift metallic interference.

– Scan and clear fault codes: Use STAR‑Xentry or Autel to read ECU errors and clear them post-repair.

– Update software / recalibrate modules: Have the dealer flash the latest firmware and calibrate sensors or modules as needed.

Cause	How to Check
Weak main battery	Measure voltage (>12.4 V) with multimeter
Weak or dead auxiliary battery	Dealer test or replace if below spec (~12 V)
Dirty radar sensor	Visually inspect & clean front bumper area
Blown fuse	Inspect and replace from fuse box diagram
Damaged wiring or connectors	Inspect under bumper & wiring loom visually
Faulty radar sensor/module	Requires diagnostic scan & professional test
Software glitch or outdated firmware	Check with dealer & update firmware

When to See a Professional

• – Warnings persist after reset and cleaning.
• – Battery diagnostics show weak cells.
• – Damage to radar sensor, module, or connectors is visible.
• – Module firmware reinstallation fails.

Action	DIY?	Professional?
Restart vehicle	✅ Yes	🚫 Not needed
Clean radar sensor	✅ Yes	🚫 Not needed
Check & replace fuses	✅ Yes	🚫 Not needed
Battery test & replacement	✅ Yes (main), ⚠️ Aux may need shop	✅ Recommended for aux
Scan & clear fault codes	⚠️ Possible (with OBD‑II scanner)	✅ Dealer can reset & reprogram
Update firmware	🚫	✅ Dealer required
Recalibrate radar sensor	🚫	✅ Dealer required
Replace sensor/module	🚫	✅ Dealer required

Case Study #1 In Mercedes C300: Mercedes collision prevention assist plus inoperative

When the dashboard displays the message Collision Prevention Assist Plus Inoperative in your Mercedes C300 or any Mercedes Vehicles, it is important not to panic. This message simply indicates that the collision prevention system is not currently operational.

Several reasons can explain this situation, ranging from minor technical issues to more complex ones, which we will explore after understanding how the Collision Prevention Assist Plus system works.

Technological advancements in the field of automotive safety have significantly enhanced safety on our roads. One of the latest and most innovative safety systems is the ‘Collision Prevention Assist Plus.’ However, there are instances where this system may be reported as inoperative, which can raise concerns among drivers.

What is Mercedes Collision Prevention Assist Plus?

How does Collision Prevention Assist Plus work?

COLLISION PREVENTION ASSIST PLUS continuously monitors the following vehicle’s safety distance, visually and audibly warns the driver of possible collisions (e.g., rear-end accidents) with other vehicles, and substantially helps mitigate the consequences of an accident. In critical situations, COLLISION PREVENTION ASSIST PLUS autonomously initiates partial braking as necessary.

At what speed is pre-collision assist active?

For this purpose, the COLLISION PREVENTION ASSIST PLUS control unit analyzes traffic conditions in front of the vehicle, the likely route, and the driver’s behavior, continually assessing the potential risk of a possible collision.

Corresponding warnings are issued within a speed range of v = 7 to 250 km/h. If the driver responds to the warning signals and brakes, they are assisted by adaptive emergency braking. The necessary reinforcement of braking force is calculated based on the situation. Autonomous partial braking is initiated if the driver does not react to the warnings. This helps to avoid or at least mitigate the consequences of a potential collision.

When an obstacle within the radar sensor’s detection zone is classified as an imminent accident (collision within less than t < 2.6 s), the COLLISION PREVENTION ASSIST PLUS control unit sends a request for warning issuance via the CAN network, including the electronic ignition switch controller and CAN user interface, to the instrument cluster.

The instrument cluster then activates the DISTRONIC warning light and directly controls the instrument cluster’s speaker (intermittent audible signal).

The COLLISION PREVENTION ASSIST PLUS control unit transmits the request for increased braking torque and reduced engine torque via the CAN peripheral, ignition contact controller, and FlexRay chassis network to the electronic stability program control unit.

The dynamic behavior regulation control unit evaluates the incoming requests, directly commands the hydraulic unit of the traction system, and transmits the request for reduced engine torque via the FlexRay chassis network, engine line controller, and CAN engine to the CDI control unit or the ME control unit.

To controle the electrical wires of the multiplexing network CAN, Flexray and LIN i recommend a Mercedes-Benz compliant multimeter and Picoscope

Show how collision prevention assists plus Mercedes A-Class work in the following video.

Case Study #2 In Mercedes CLS W218: Mercedes collision prevention assist plus inoperative

A 2016 Mercedes-Benz CLS 550 4MATIC was brought to our shop with a host of problems. The most pressing issue reported was the inoperative Collision Prevention Assist Plus system. The previous mechanic had replaced several components, including the ESP (Electronic Stability Program), ECU/control unit, and speed sensors, but the problem persisted.

Diagnosis Process

Given the extensive list of components already replaced, we decided to start our diagnosis by focusing on the ESP system. The ESP is critical as it monitors the vehicle’s speed and stability. Upon closer inspection, we discovered that the speed sensors previously installed were not original parts. These non-original sensors were providing incorrect values, even when the vehicle was stationary.

To resolve this, we replaced both front speed sensors with genuine Mercedes parts. After the replacement, the erroneous values returned to zero, as expected, indicating that the speed sensors were now functioning correctly.

Tracing Collision Prevention Assist Issue

With the speed sensor issue resolved, we shifted our focus to the Collision Prevention Assist system. We conducted a thorough inspection of the system, checking for any visible damage, wire melting, bridged connections, or water damage to the connectors. However, we found no issues in these areas.

Next, we decided to check the auxiliary battery, which controls the ECO Start/Stop function. We discovered that the auxiliary battery had a voltage of only 0.47 volts. This low voltage had caused the FRONT SAM (Signal Acquisition Module) to have a short circuit to ground. The auxiliary battery was essentially dead, likely for years, which explained the persistent issues with the Collision Prevention Assist system.

Solution Implementation

To address this, we replaced the auxiliary battery with a new one. After the replacement, we reset the system and performed a series of tests to ensure that all related systems, including the Collision Prevention

Mercedes collision prevention assist plus sensor location

Where is the forward collision sensor located?

The location of the Collision Prevention Assist Plus sensor depends on the type of Mercedes and can be found in two different places. It may be positioned behind the Mercedes emblem or in the right-side bumper, as illustrated in the image below.

The variability in location ensures accurate detection of potential hazards and contributes to enhancing the overall safety of the vehicle.

Show in the following video how to replace the collision prevention assist plus sensor

How do you clean a collision sensor?

Steps to Clean a Collision Sensor

1. 1. Locate the Sensors
   • – Collision sensors are typically located in the front grille, rear bumper, or side mirrors, depending on the system. Refer to your owner’s manual for exact locations.
2. 2. Turn Off the Vehicle
   • – Ensure the car is turned off to prevent accidental activation of systems during cleaning.
3. 3. Inspect for Debris
   • – Check the sensor area for dirt, dust, bugs, or grime that might interfere with its operation.
4. 4. Clean Gently
   • – Dampen the microfiber cloth with a mild automotive cleaner or soapy water. Avoid harsh chemicals that might damage the sensor housing.
   • – Gently wipe the sensor and the surrounding area in a circular motion to remove dirt.
5. 5. Use a Soft-Bristle Brush for Stubborn Dirt
   • – For dried-on debris, use a soft-bristle brush to loosen the material carefully. Avoid applying excessive pressure.
6. 6. Dry the Area
   • – Use a dry microfiber cloth to remove any remaining moisture. Ensure the sensor area is completely dry before turning the vehicle back on.
7. 7. Compressed Air for Hard-to-Reach Spots
   • – If dirt is stuck in crevices, use compressed air to blow it out. Keep the nozzle at a safe distance to avoid damaging the sensor.
8. 8. Check for Proper Alignment
   • – Ensure the sensor is not misaligned or loose. If it appears damaged or shifted, professional calibration may be necessary.

How to activate collision prevention assist plus?

To activate Collision Prevention Assist Plus in a Mercedes-Benz vehicle, you typically need to follow these steps:

1. Start the vehicle: Ensure the engine is running and the vehicle is in operation.

2. Navigate to the vehicle settings menu: Depending on the model and year of your Mercedes-Benz, you may access the settings menu through the infotainment system display or the multifunction steering wheel controls.

3. Locate the “Assistance Systems” or “Safety Systems” menu: Within the settings menu, look for options related to assistance or safety systems. This is where you’ll find the settings for Collision Prevention Assist Plus.

4. Enable Collision Prevention Assist Plus: Once you’ve located the appropriate menu, you should see an option to enable or activate Collision Prevention Assist Plus. Select this option and follow any on-screen prompts to confirm your selection.

5. Adjust settings if necessary: Some Mercedes-Benz vehicles allow you to adjust the sensitivity or behavior of Collision Prevention Assist Plus. If desired, you can customize these settings to suit your preferences.

6. Confirm activation: After enabling Collision Prevention Assist Plus, ensure that it is activated and functioning properly by checking for any indicator lights or messages on the instrument cluster or infotainment display.

It’s important to note that the exact steps for activating Collision Prevention Assist Plus may vary depending on the specific model and year of your Mercedes-Benz vehicle.

For precise instructions tailored to your vehicle, refer to the owner’s manual or consult a Mercedes-Benz dealership or authorized service center. They can provide guidance and assistance with activating and configuring Collision Prevention Assist Plus to best suit your needs.

Show how to activate collision prevention assist plus in Mereceds E-Class in the following video

Frequently Asked Questions