Mercedes-Benz Active Brake Assist Function Limited – Complete Guide
Active Brake Assist is a vital safety feature in modern Mercedes-Benz vehicles. It helps prevent collisions by warning the driver and automatically applying the brakes if necessary. When this system has issues, the driver may see a warning message such as ‘Active Brake Assist Function Limited’ on the instrument cluster. This article provides a complete guide on how Active Brake Assist works, the common error messages, possible causes of malfunction, diagnosis steps, repair solutions, and the calibration process.
What is Active Brake Assist?
Active Brake Assist is part of the Mercedes-Benz safety suite. It uses radar sensors located at the front of the vehicle and sometimes a forward-facing camera to detect obstacles and traffic ahead. If the system determines a collision risk, it alerts the driver visually and audibly. If the driver does not respond, Active Brake Assist automatically engages partial or full braking to mitigate or avoid an accident.
Common Error Messages in the Instrument Cluster
When Active Brake Assist malfunctions, the following messages may appear:
- Active Brake Assist Function Limited
- Collision Prevention Assist Inoperative
- Brake Assist Malfunction
Causes of Active Brake Assist Errors
The most common causes of system limitation or failure include:
- Radar sensor obstruction – dirt, snow, or damage to the front bumper blocking the sensor.
- Misalignment – after a collision, bumper replacement, or improper repair work.
- Wiring issues – damaged or corroded wiring to the radar unit or brake assist ECU.
- Radar sensor failure – electronic fault inside the radar module.
- Control unit malfunction – issues in the Active Brake Assist ECU.
- Environmental factors – heavy rain, fog, or extreme sunlight temporarily limiting the system.
How to Diagnose Active Brake Assist Errors
Diagnosis starts with a visual inspection of the radar sensor, usually located in the front grille or bumper, to check for dirt, debris, or physical damage. Next, use a diagnostic tool such as Mercedes Xentry, Vediamo, DTS Monaco, or the MBCOD Box (with ELM327) to scan for fault codes. Common codes may include C1100, C1101, or U0235, which point to radar communication or sensor issues.
How to Fix Active Brake Assist Errors
To fix Active Brake Assist issues, follow these steps: - Clean the front radar sensor area to remove dirt or obstructions.
- Perform a diagnostic scan to read and clear fault codes.
- Repair or replace damaged wiring or connectors leading to the radar sensor.
- Replace the radar sensor if confirmed faulty.
- Reset or reprogram the Active Brake Assist ECU if necessary.
- Calibrate the radar sensor after replacement or bumper repair.
Calibration of Active Brake Assist
Calibration is required whenever the radar sensor is replaced, adjusted, or after any collision repair affecting the front bumper. Mercedes-Benz uses special calibration targets and diagnostic tools such as Xentry for precise alignment. The vehicle must be positioned on a level surface with calibration boards placed at exact distances. The diagnostic tool then runs a guided process to ensure the radar detects obstacles correctly. Incorrect calibration can cause false warnings, limited functionality, or complete system failure.
When to Contact a Specialist
If the warning ‘Active Brake Assist Function Limited’ persists despite basic troubleshooting, it is best to contact a specialist. At MBCOD in Dubai, we provide expert diagnostics, radar sensor replacement, ECU programming, and full calibration services for Active Brake Assist and other Mercedes-Benz safety systems. Our mobile service also covers Dubai, Abu Dhabi, Sharjah, and Ajman.
Conclusion
Active Brake Assist is a critical safety system designed to protect drivers and passengers from accidents. When it shows ‘Function Limited,’ it should be addressed immediately. With the right diagnosis, repairs, and calibration, the system can be restored to proper working condition. Trust MBCOD in Dubai and the UAE for professional Mercedes-Benz diagnostics, ECU services, and radar calibration.