Active Brake Assist Functions Limited Diagnosis Mercedes-Benz

This article explains 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 others.

What does Active Brake Assist Functions Limited mean?

Active Brake Assist is a collision prevention system that aims to reduce the risk of a collision by warning the driver of vehicles, cyclists, or pedestrians and reduces the likelihood of injury in case of such a crash. The  Active Brake Assist Functions Limited message indicates that the brake assist function is not working correctly. Active Brake Assist Functions Limited warning is usually caused by a sensor that does not function properly or low voltage issues.

The most likely cause is that the front sensor is covered by dirt and snow. The sensor might also lose power, the engine check light might come on, or the vehicle could go into limp-home mode. If you experience one of these things, pull to a safe area and restart the vehicle.

If that doesn’t work, try cleaning the front grill, the left and right side of the front bumper, and the top center of the windshield; in most cases, this fix will clear the messages. If your vehicle battery is getting too weak or if there is a software issue with the Distornic Plus control unit, the active brake assist warning may come up.

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

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

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

Can you drive with the Active Brake Assist light on?

Yes, you can, but it is not recommended. The vehicle will not brake for you in emergencies. If the problem is too severe, the vehicle may go into limp mode, which means it will only drive at speeds up to 40 mph. In limp mode, the car is supposed to be driven to the nearest auto mechanic or home.

Possible Issues

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 also a common fault for Mercedes-Benz cars when you drive in the 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 car 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, direct sunlight, or 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 other objects hide pedestrians
• 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
• An incorrect windshield was installed

Possible Fixes

If your Mercedes-Benz is still under warranty, take the car to the dealer and let the dealer resolve the issue. If your car is out of warranty, the following suggestions will usually help resolve the Active Brake Assist Functions Limited warning.

1. Remove all dirt and snow from the front windshield, grill, and bumper.
2. Deactivate the transport mode and drive the vehicle for approx. 10 km (6 mi). The DTC then switches to passive, and the sensor can be calibrated. The Active Brake Assist should recalibrate automatically, and the error clears if the code is due to a dirt issue.
3. Disconnect the 12-volt battery for 15 minutes and reconnect it.
4. If the problem persists, read the fault codes.

For more help, see the Message (yellow) in IC: “Active Brake Assist currently restricted, see operator’s manual”.

How does  Active Brake Assist work?

Active Brake Assist with Cross-Traffic Function is designed to avoid an impact entirely with pedestrians, animals, or other objects and vehicles ahead, reduce damage, or minimize its harmful effects if the driver’s reaction to an emergency event is delayed. This way, if the driver doesn’t respond by applying sufficient deceleration pressure to the accelerator pedal to avoid the collision altogether, or if Assist of this type applies the maximum braking pressure itself if this also fails to prevent an accident, Active Brake Assist triggers autonomous emergency braking.

The big question is, ‘Is my Mercedes equipped with active brake assist?’ If you own a Mercedes-Benz built after 2014 with an active brake assist and have driven for more than a day, the car will warn you or even apply the brakes on you at least once. If you are uncertain, call any Mercedes-Benz dealer and give them your VIN to be decoded, and they can tell if your vehicle has an active brake assist installed. Autonomous braking can be initiated in critical situations if you do not react to the warning. 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 do you 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 the vehicle will not brake automatically with brake assist off to avoid a collision.

Active Brake Assist Disabled

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.
• The vehicle can no longer detect an obstruction, block, or barrier.

Fault Codes

Possible fault codes include:

• C163691 The calibration of the control unit ‘DISTRONIC’ is malfunctioning. The parameter is outside the permissible range.
• C174AFB The learned value of an operating parameter of the control unit ‘COLLISION PREVENTION ASSIST’ is outside the valid range.
• C174AFA The learned value of an operating parameter of the control unit ‘COLLISION PREVENTION ASSIST’ is outside the valid range.
• C163691 The calibration of the control unit ‘DISTRONIC’ is malfunctioning. The parameter is outside the permissible range.
• C174AFB The learned value of an operating parameter of the control unit ‘COLLISION PREVENTION ASSIST’ is outside the valid range.
• C174AFA The learned value of an operating parameter of the 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.

Troubleshooting

This error message directs the driver to the owner’s manual for more information, but unfortunately, there is little helpful information. To find out why the Active Brake Assist malfunction is displayed 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 using the YOUCANIC full system scanner.

Frequently Asked Questions

What does Active Brake Assist Limited mean?

• This setting can help prevent or avoid vehicle collisions with the car in front, parked cars, cyclists, and people crossing the street.

Can you turn off Active Brake Assist?

• You can turn off vehicles with active safety and driver assist systems like Automatic Emergency Braking.

What are the three types of brake assist systems?

• Vacuum Brake Booster
• Vacuum Pump
• Hydraulic Brake Boosters

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