6 Causes of Mercedes Active Brake Assist Functions Limited

Safety is a top priority for all car manufacturers, and advanced driver-assistance system like Active Brake Assist plays an important role in the driver’s safety.

However, there may be malfunctions, and you may encounter warning messages such as “Active Brake Assist Function Limited.” It is a clear sign that something has gone wrong, and your safety with the car is compromised.

This article will shed light on the causes and provide practical solutions to get your vehicle brake system functioning properly. We will also answer some important questions we think you may have. Let’s get started!

What does Active Brake Assist Functions Limited mean?

An active brake assist functions limited is a warning message that is exclusive to Mercedes vehicles. 

When this warning message comes up, it shows that the active brake assist is not working correctly. Damaged sensors could majorly cause this.

Active brake assist is a feature in most Mercedes models that helps reduce the risk of collision. When the risk arises, a warning is triggered. And once the driver hits the brake, the assistance is activated in response to the brake pressure. 

The automatic braking will be activated if the driver does not respond during a potential collision. 

In cases where the active brake assist function limited warning is displayed, the function will not be activated during the risk of collision. It is why you need to get the problem fixed quickly. 

Causes of Active Brake Assist Not Available

The common cause of active brake assist function limited is anything to affect related sensors. Your vehicle uses sensors like video, radar, infrared, or ultrasonic. These sensors scan for possible objects that may come in the way of your car when driving. 

Objects will not be detected if these sensors become damaged or do not work. It could increase your chances of colliding with another car, bike, and more. 

Below are factors that could cause an active brake assist function limited warning message. 

  1. Damaged radar sensors 
  2. Dirt or any form of contamination on the sensors 
  3. Wiring damaged wiring harness 
  4. Issues with the Power Control Module 
  5. Low battery voltage 
  6. Complex traffic situation 

What are the Possible fixes?

How to fix an Active Brake Assist Functions Limited depends on the cause of the problem. You will need a proper vehicle diagnosis to determine the cause before fixing the problem. 

Once you can figure out the cause of the warning message, then you can proceed with the following fixes. 

1. Remove all dirt or any form of contamination. 

One of the common causes of the warning message is a dirty windshield, grill, and bumper. To be able to get the function working again, try and clean any form of contaminant in these areas. Ensure it is cleaned properly, and then put it on your vehicle. 

The message should not appear after cleaning; if it does, it is not due to dirt or a more severe issue. 

2. Check and fix the wiring issue.

Another problem that could cause the warning message to come up is a damaged wire related to the sensor radar. The good thing is that your vehicle’s manual gives you a proper diagram of all the wires of your car. 

Consult your vehicle’s manual if you do not have one. You can easily download it online. Search for the wires related to the sensor radars and check for any form of disconnection or damage. 

If you find damage, you must change the wires completely. Trying to cut and join could be dangerous and lead to short circuits.

3. Replace the sensor radar.

Once you can diagnose properly and determine that the sensor radar is damaged, you may find need to replace it. If you have yet to gain any prior mechanical knowledge, you may need assistance from a mechanic. 

The radar sensor is located at the front of your vehicle and behind the grill. This strategic location allows it to sense objects properly. To access the sensor, you must remove the bumper, grill, or windshield.  

Once you can remove these components, you can remove the sensor and create a new one. 

4. Fix low voltage problems

One of the common causes of this warning message is when associated components like the sensors and PCM are not receiving adequate voltage. Factors that could cause this problem could be related to a faulty battery or wiring issues. 

To solve the problem, you must first test your vehicle’s battery to ensure it works properly. A way to know if your battery is bad is to look at other symptoms that come with it—for instance, problems with ignition, dim light, slow cranks, etc. 

Also, check the battery connections and other related connections and have them fixed immediately. 

Battery problems can cause the entire vehicle to malfunction, and the battery should be replaced if entirely worn out. If you just have a dead battery, make sure to charge it.

5. Powertrain control module fix   

The powertrain control module is the computer system that controls a majority of the functions in your vehicle, including the active brake assist. Once there is a PCM problem, most of your vehicle’s components will also fail. 

Sometimes, your PCM failure could be due to software issues and may need to be reprogrammed. You might need a reflashing process done on your PCM. This is when you electronically transfer approved files from your vehicle manufacturer’s site into the diagnostic connector and then into the PCM. 

You should replace the module if the PCM is still bad and not responding to the reprogramming. 

Due to how delicate the PCM is, it is best to let a professional handle this process. It will help prevent further damage to your vehicles. 

Can you drive with an Active Brake Assist Light On?

You can still drive your vehicle with an active assist light on. However, in some severe cases, your car might go into mode, which means you cannot speed more than 40 mph.  

If you have the active brake assist light on and want to avoid going into limp mode, you can quickly turn off the function via the steering wheel; control. To do that, follow the instructions below.

  1. Navigate to the vehicle setting
  2. Scroll down to assistance 
  3. Find active brake assist and select
  4. You will find one of the settings that display options high, medium, low, or off.
  5. Select off to disable.  


The “Active brake assist function limited” error message should never be neglected regarding your safety. We recommend that you try to identify the root cause of The message and then apply possible fixes to them. Remember always to consult a professional mechanic if you need help trying to fix your vehicle.

  • Edmond Davis

    I'm Edmond Davis, an automotive expert with years of experience in vehicle repair, performance, and safety. I graduated from the University of Michigan with a degree in Automotive Engineering and have worked with major companies like Ford, GM, and Chrysler. I'm a trusted source of information for anyone looking to learn more about cars or improve their driving experience.

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