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Active Brake Assist Functions Limited Diagnosis Mercedes-Benz

In this article, we explain what Active Brake Assist Functions Limited See Operator's Manual means, why this light comes on, common causes and how to fix it. This issue can affect any Mercedes-Benz vehicle equipped with Brake Assist including models in the GLA-Class, S-Class, Sprinters, and other models.

Active Brake Assist Functions Limited Diagnosis Mercedes-Benz
ESTIMATED TIME 30 min - 2 hr
DIY COST $0-$1450
DIFFICULTY Intermediate

What does Active Brake Assist Functions Limited mean? 

Active Brake Assist can help you minimize the risk of a collision with vehicles, cyclists, or pedestrians. If an accident cannot be avoided, the car will try to reduce the impact of such a collision.

The  Active Brake Assist Functions Limited message indicates that the brake assist function is not working correctly. Active Brake Assist Functions Limited warning is in most cases caused due to a sensor that does not have full sight.

Mercedes active brake assist functions limited

The most common reason for this is that the front sensors are covered with dirt or snow, thus obscuring the range.

In some cases, the vehicle may lose power, no longer respond to the gas pedal, turn the engine light on, and even go into limp mode. If you experience these symptoms, pull over to a safe location and restart the vehicle.

If that doesn't work, try cleaning the front grill and the top center of the windshield; in most cases, this fix will clear the messages. 

If the active brake assist warning comes up in the morning, it is most likely your vehicle battery is getting too weak or there is a software issue. 

Depending on the Mercedes-Benz model, the warning message may say: 

  • Active Brake Assist Functions Limited See Owners Manual
  • Active Brake Assist is Malfucntiong. Consult a qualified specialist workshop
  • Active Brake Assist Inoperative See Oper. Manual
  • Active Brake Assist Functions Currently Limited
  • Active brake assist not available 

If the message continues, you will need to read the fault codes by connecting a Full System Scanner into the OBD-II port under the dashboard.

Can you drive with the Active Brake Assist light on? 

Yes, you can, but the vehicle will not break for you in emergencies. 

If the problem is too serious, the vehicle may go into limp mode, which means it will only drive at speeds up to 40 mph. In limp mode, the vehicle is supposed to be driven to the nearest auto mechanic or home. 

Possible Issues

brake assist sensor location

As stated earlier, in most cases, Active Brake Assist Functions Limited means the brake assist 'sensor' at the front of the car is dirty and therefore can't sense distances to the object at the front. It is a common fault for Mercedes-Benz cars when there is snow.

Try clearing the snow from the front of the vehicle and the windshield, and most likely remove errors from the dashboard. Or even better, take the vehicle for an exterior wash. 

Other possible causes of  Active Brake Assist Functions Limited include:

  • Loose connection
  • Coding issue
  • Low battery voltage
  • Damaged radar sensor
  • Incorrect coding

According to the owner's manual, the system may be impaired or may not function in the following situations:

  • In snow, rain, fog, heavy spray, if there is glare, in direct sunlight or greatly varying ambient light.
  • If the sensors are dirty, fogged up, damaged, or covered
  • If the sensors are impaired due to interference from other radar sources, e.g., strong radar reflections in parking garages
  • If a loss of tire pressure or a defective tire has been detected and displayed
  • In complex traffic situations where objects cannot always be identified
  • If pedestrians or vehicles move quickly into the sensor detection range
  • If pedestrians are hidden by other objects
  • If the typical outline of a pedestrian cannot be distinguished from the background
  • If a pedestrian is not detected, e.g., due to unique clothing or other objects
  • On bends with a tight radius
  • Pins for the throttle pedal harness can get damaged
  • Active brake assist may require calibration
  • Incorrect windshield was installed

Possible Fixes

If your vehicle is still under warranty, take it to any Mercedes-Benz dealer.

If your vehicle is not under warranty, the following tips will, in most cases, help fix the Active Brake Assist Functions Limited warning. 

  1. Remove all dirt and snow from the front windshield, grill, and bumper. 
  2. Activate transport mode and drive vehicle for approx 10 km. The DTC then switches to passive, and the sensor can be calibrated. The Active Brake Assist should recalibrate automatically, and the error clears on its own if the code was due to a dirt issue. 
  3. If the problem persists, read the fault codes.

How does  Active Brake Assist work? 

Active Brake Assist aims to prevent accidents or minimize the damage with pedestrians, animals, objects, and vehicles in front. Active Brake Assist applies additional braking pressure if the driver doesn't press the brake pedal hard or fast enough to avoid a collision. If the driver does not respond at all, Active Brake Assist initiates autonomous emergency braking.

A question that we get asked often is: "Does my Mercedes have active brake assist?" - If your Mercedes-Benz has active brake assist and you have driven the vehicle for a few days you will know because the vehicle will most likely apply the brakes for you at least once. If you are still not sure, call up any Mercedes-Benz dealer, give them your VIN number, and they can let you know if your vehicle has active brake assist installed. 

If you do not react to the warning, autonomous braking can be initiated in critical situations. In especially critical cases, Active Brake Assist can initiate autonomous braking directly.

The distance warning function issues a warning in the following situations:

  • From approximately four mph (7 km/h), if your vehicle is close to a car or pedestrian, you will hear an intermittent warning tone, and the distance warning lamp lights up in the instrument cluster.

Brake immediately or take evasive action, provided it is safe to do so, and the traffic situation allows this.

Active Brake Assist consists of the following functions:

  • Distance warning function
  • Autonomous braking function
  • Situation-dependent braking assistance
  • Vehicles with Driving Assistance Package: Evasive Steering Assist

How to turn off active brake assist?

The active brake assist can be turned off via the steering wheel controls

  1. Go to vehicle settings
  2. Then scroll down to Assistance
  3. Scroll down to Active Brake Assists. 
  4. Choose one of the settings. High, Medium, Low, or OFF. 
  5. To disable brake assist select OFF. 

Note that with brake assist off, the vehicle will not brake automatically to avoid a collision.   

Active Brake Assist Disabled (by design)

Active Brake Assist may cancel the brake application when one of the following conditions is fulfilled:

  • You maneuver to avoid the obstacle.
  • There is no longer a risk of collision.
  • An obstruction, block, or barrier is no longer detected by the vehicle.

Fault Codes

Possible fault codes include: 

  • C163691 The calibration of the control unit 'DISTRONIC' has a malfunction. The parameter is outside the permissible range. 
  • C174AFB The learned value of an operating parameter of control unit 'COLLISION PREVENTION ASSIST' is outside the valid range. 
  • C174AFA The learned value of an operating parameter of control unit 'COLLISION PREVENTION ASSIST' is outside the valid range. 
  • C163691 The calibration of the control unit 'DISTRONIC' has a malfunction. The parameter is outside the permissible range.
  • C174AFB The learned value of an operating parameter of control unit 'COLLISION PREVENTION ASSIST' is outside the valid range. 
  • C174AFA The learned value of an operating parameter of control unit 'COLLISION PREVENTION ASSIST' is outside the valid range. 

This is a partial list of  Active Brake Assist / Active Distance Assist DISTRONIC-related fault codes. Other fault codes may be stored in the Active Brake Assist module. 

Further Troulbeshoting

This error message directs the driver to find more information in the owner's manual, but unfortunately, there is little helpful information in the manual.

To find out why the Active Brake Assist malfunction is displaying on the instrument cluster, you must read the fault codes from the  Active Brake Assist / Active Distance Assist DISTRONIC control module. Follow this guide on how to read Mercedes-Benz fault codes. 

 

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