Mercedes ‘Collision Prevention Assist Plus inoperative’ Fix

Mercedes ‘Collision Prevention Assist Plus inoperative’ Fix

One safety feature you can find on Mercedes models is the ‘Collision Prevention Assist Plus’ system.

But what happens when you see a message saying ‘Collision Prevention Assist Plus inoperative’?

This is more common than you might think, and often, it’s something you can address.

In this article, we will discuss what this safety feature means, why the error pops up, and, most importantly, how you can fix it.

Quick Answer: A ‘Collision Prevention Assist Plus inoperative’ means that a collision prevention safety feature on your Mercedes is not functioning. It is most often caused by dirty sensors or a low battery voltage.

How do you fix this error message?

Since you’re probably most interested in a fix, let’s start with that.

The most common causes of a ‘Collision Prevention Assist Plus inoperative’ message are a weak battery, dirty sensors, or defective sensor modules.

Here are the steps you should take to find the cause of the error message (and fix it):

  1. Clean the sensors.
  2. Measure the battery voltage.
  3. Reset the Collision Prevention Assist Plus system.
  4. Check your aftermarket modifications.
  5. Check for external disruptors (electromagnetic radiation or extreme temperatures).
  6. Replace the radar sensor modules.

1. Clean the sensors

Your Mercedes’ Collision Prevention Assist Plus system relies on sensors to detect potential hazards.

These sensors can get dirty, especially if you’re driving through snow, ice, or even just a lot of dirt and grime.

Therefore, a possible cause of the system being inoperative is that your radar sensors are dirty, especially if you see the message in combination with any of these warnings:

Considering it would be the easiest fix, cleaning all the radar sensors is the first thing you should try.

Here are all the radar sensor and camera locations:

  • Windshield-mounted sensor.
  • Grill-mounted camera.
  • Emblem sensor. (Most important)
  • Bumper-mounted sensors.
  • Side mirror-mounted sensor/camera.

You should thoroughly clean the emblem sensor, as this is the location of the radar sensor used for the Collision Prevention Assist Plus system.

Here are some images that make it clear where you can find all the sensors and cameras:

Mercedes windshield mounted sensor location
Windshield-mounted sensor location
Mercedes grill-mounted camera location
Grill-mounted camera location
Mercedes emblem sensor location
Emblem sensor location
Mercedes bumper-mounted sensor locations
Bumper-mounted sensor locations
Mercedes side-mirror camera sensors location
Side-mirror camera sensor location

Once the sensors are clean, take your car for a test drive and see if the message disappears.

Are you still seeing the message? In that case, it might be caused by something different than dirty sensors (unfortunately!)

2. Measure the battery voltage

Believe it or not, a weak battery is a very common cause of the ‘Collision Prevention Assist Plus inoperative’ message.

Your Mercedes’ safety systems rely heavily on a strong, healthy battery. When the battery starts to lose its charge, these systems might not function properly.

A Mercedes battery should measure at least 12.0V when the car is not running.

If your battery is weak, you might also see the ‘Stop Vehicle Leave Engine Running‘ or ‘Stop Vehicle Shift to ‘P’ Leave Engine Running’ message on your dashboard:

Mercedes ‘Stop Vehicle Leave Engine Running’ Meaning & Fix

You can test your Mercedes battery using a multimeter (a device used to measure electrical voltage, current, and resistance).

Measuring a car battery - Illustration

Here is a step-by-step process to test your Mercedes battery:

  1. Safety first: Make sure the car is off and the keys are out of the ignition.
  2. Locate the battery: Open the hood and locate your Mercedes battery. It’s usually easy to find:
Mercedes battery location
  1. Check for visible issues: Before using the multimeter, visually inspect the battery for any signs of damage or corrosion on the terminals.
  2. Set up your multimeter: Set your multimeter to the DC Voltage setting, marked as “V” with a straight line (for direct current voltage).
  3. Connect the multimeter: Attach the red lead of the multimeter to the positive terminal of the battery (usually marked with a + sign or red in color) and the black lead to the negative terminal (marked with a – sign or black in color).
  4. Read the voltage: A healthy Mercedes battery should show a voltage between 12.6 to 12.7 volts. If it’s below 12.4 volts, the battery is weak and may have triggered the ‘Collision Prevention Assist Plus inoperative’ message. If it’s below 12.0 volts, the battery definitely needs a replacement.

If you don’t want to test the battery yourself, most auto shops will also do a quick check for you.

If the battery is indeed running low, you have two options: charge it or replace it.

Charging might be a good short-term fix, but if your battery is more than four years old, replacement is the best long-term solution.

Tip: You can learn how to replace a Mercedes battery by yourself here!

3. Reset the Collision Prevention Assist Plus system

Sometimes, a reset can help clear the error message.

Here is how to reset the Collision Prevention Assist Plus system:

  1. Take your key and switch the ignition to position 1. This turns on the electrical systems but does not turn on the engine. If you have a start/stop button, push it once without holding your foot on the brake.
  2. Navigate to the ‘Trip’ menu. Use the ‘up’ and ‘down’ arrows on your steering wheel to navigate to the ‘Trip’ menu, where you can see the mileage on your car:
Mercedes Trip menu
  1. Push and hold the ‘OK’ and ‘Phone’ buttons on your steering wheel for three seconds. This will bring up the service interface.
  2. Navigate to ‘Assist Plus.’ Use the ‘up’ and ‘down’ arrows on your steering wheel. Click ‘OK’ on your steering wheel to enter the menu:
Mercedes Assist Plus menu
  1. Switch the key to position 2. This will unlock the Assist Plus menu.
  2. Press ‘OK.’ You will see a ‘Service A’ message. Press the ‘OK’ button on your steering wheel:
Mercedes Service A menu
  1. Confirm the reset. Select the ‘Service’ option and confirm it using the ‘OK’ and ‘up’/’down’ arrow buttons on your steering wheel.

That’s it! The reset is done.

4. Check your aftermarket modifications

This one will be quick and simple.

If you did any aftermarket modification to the front of your car, like a new front bumper, it may be the cause of your error message.

The reason for this is simple: aftermarket modifications can partly or fully block the radar sensors used for the Collision Prevention Assist Plus system, making it inoperative.

5. Check for external disruptors

Sometimes, the issue isn’t with your car but with the environment around it.

External factors like electromagnetic radiation or extreme temperatures can interfere with the radar sensors.

Consider where you park and drive your Mercedes when the error message shows up.

Is your car exposed to extreme heat or cold?

Are you near high-voltage power lines, radio towers, or other sources of electromagnetic radiation?

A radio tower emitting electromagnetic radiation illustration

Try parking in a different location or driving a different route to see if the message clears. If it does, you’ve found the external disruptor.

6. Replace the radar sensor modules

If all else fails, you may have to replace the radar sensor modules.

The radar modules in your car use a mix of analog and digital technologies.

They can be sensitive to rain intrusion or voltage swings.

A defective radar module not only affects the Collision Prevention Assist Plus system but can also prevent your car from entering low-power sleep mode, leading to battery drain while parked.

So, what to do?

The first step is to get a clear picture of what’s going on.

You can scan your car for a list of faults. This can be done with an OBD-II scanner if you have one or at a Mercedes-Benz service center.

If the radar modules are indeed the problem, it’s best to book a visit with a Mercedes-Benz specialist. They can confirm the issue and replace the radar modules if necessary.

What is ‘Collision Prevention Assist Plus’?

Collision Prevention Assist Plus is a safety feature on Mercedes-Benz vehicles that consists of a distance warning function, an autonomous braking function, and Adaptive Brake Assist.

If there is a risk of running into the vehicle ahead, Collision Prevention Assist Plus minimizes this risk or, at least, reduces the effects if a collision does occur.

Mercedes Collision Prevention Assist Plus feature

If the system senses a risk of a collision, it activates visual and audible warnings.

If you miss these warnings, the system initiates autonomous braking. This means it can automatically apply the brakes to help prevent a collision.

However, if you do react and start braking, the Adaptive Brake Assist feature kicks in, adjusting the braking force to match the situation.

In cases where strong braking is needed, the system activates PRE-SAFE. This feature is designed to protect passengers in case a collision is unavoidable.

Collision Prevention Assist Plus works at speeds of up to 124 mph for moving objects or 31 mph for stationary objects.

How do you deactivate Mercedes Collision Prevention Assist Plus?

It is possible to deactivate Collision Prevention Assist Plus. Here are the steps to follow:

  1. Press the ‘Home’ button on the steering wheel to open the menu list.
  2. Press the ‘up’ or ‘down’ arrow buttons on the steering wheel to select the ‘Settings‘ menu.
  3. Press the ‘OK’ button on the steering wheel to open the ‘Settings’ menu.
  4. Press the ‘up’ or ‘down’ arrow buttons to select the ‘Drive Assist‘ menu.
  5. Press the ‘OK’ button to confirm.
  6. Press the ‘up’ or ‘down’ arrow buttons to select the ‘Collision Prevention‘ menu.
  7. Press the ‘OK’ button to confirm.
  8. To activate or deactivate Collision Prevention Assist Plus, press the ‘OK’ button.

When Collision Prevention Assist is deactivated, you will see its symbol display on your dashboard:

Mercedes Collision Prevention Assist off symbol


We’ve covered quite a bit about the Collision Prevention Assist Plus system in your Mercedes, haven’t we?

You now know that the most common causes for this message can be something as simple as dirty sensors or a weak battery.

And with the steps we’ve discussed, like cleaning the sensors, checking the battery voltage, or resetting the system, you’re well-equipped to tackle these issues.

Of course, if these steps don’t do the trick, reaching out to a professional is always a wise decision. They can delve deeper into more complex issues like radar sensor module faults.

Safe driving, and enjoy the journey in your Mercedes!

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