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Pre Safe Functions Limited Mercedes: Meaning, Causes & Real Fixes
When your Mercedes displays the warning “Pre Safe Functions Limited”, it means that one or more sensors within the Pre-Safe safety system are malfunctioning, obstructed, or not communicating correctly, limiting the system’s ability to prepare for a potential collision.
The Pre-Safe system, introduced by Mercedes-Benz as part of its Intelligent Drive safety suite, anticipates accidents using a network of radar sensors, cameras, and control modules. It automatically initiates preventive actions tightening seat belts, adjusting seat positions, closing windows, or priming brake pressure to protect occupants before impact.
For a complete guide to all Mercedes ADAS (Advanced Driver Assistance System) faults including radar, blind spot, and Parktronic issues visit our expert hub:
Mercedes Driver Assistance System Faults: Radar, Blind Spot & Parktronic Fixes
How the Mercedes Pre-Safe System Works
The Pre-Safe system continuously monitors vehicle dynamics and the environment through:
- – Front radar sensors (mounted behind the bumper or grille)
- – Camera control unit (analyzing lane and object data)
- – Yaw, steering angle, and acceleration sensors (from ESP and BAS systems)
If the system detects an imminent impact or instability, it triggers pre-crash protection functions such as:
- – Seat belt pre-tensioning
- – Seatback and headrest adjustment
- – Window and sunroof closure
- – Brake assist preparation
When the Pre-Safe Functions Limited message appears, one of these modules has either lost communication, received inconsistent data, or is temporarily restricted by voltage or environmental factors.
Common Causes of the “Pre Safe Functions Limited” Warning
| Cause | Typical Symptoms | Quick Fix / Diagnostic Step |
|---|---|---|
| Dirt, snow, or ice on radar sensors | Warning after harsh weather | Clean front bumper, grille, and camera lenses |
| Faulty or corroded connectors | Persistent warning even after cleaning | Inspect radar harness for water intrusion or damaged pins |
| Malfunctioning camera or radar control unit | Warning reappears after restart | Scan system with XENTRY; check for DTCs such as U019B87 or B220600 |
| Low voltage or weak auxiliary battery | Intermittent Pre-Safe faults and ESP errors | Test battery voltage; should read 12.4–12.8 V at rest |
| Outdated camera control unit software | Random false Pre-Safe alerts | Perform control unit update with latest Mercedes firmware |
Step-by-Step Diagnostic Flow
- 1. Clean all radar and camera sensors especially in the bumper, grille, and windshield area.
- 2. Cycle the ignition restart vehicle and check if the warning clears.
- 3. Perform a full system scan using Mercedes XENTRY Diagnostics or equivalent tools.
- – Check for stored or current fault codes in the Pre-Safe, ESP, and Radar modules.
- 4. Inspect harness connectors and grounds look for moisture, corrosion, or chafed wiring.
- 5. Update or re-flash the Camera Control Unit (CCU) if DTCs persist.
- 6. Verify voltage stability on the auxiliary battery and CAN communication integrity.
Case Study: Pre Safe Functions Limited, Camera Control Unit Fault in Mercedes-Benz E-Class
Customer Complaint
A customer brought in a Mercedes-Benz E-Class (W213) with a recurring “Pre Safe Functions Limited” message that appeared randomly while driving, even in clear weather. The vehicle’s adaptive braking and lane assist systems were otherwise functioning normally.
Initial Inspection
A visual inspection revealed no dirt or blockage on the front radar or windshield camera areas. The main and auxiliary batteries both tested within acceptable voltage levels. Since environmental and voltage causes were ruled out, attention turned to the camera control unit (CCU) a critical component responsible for processing visual input for both Pre-afe and Collision Prevention Assist systems.
Diagnostic Process
Using XENTRY Diagnostics, the technician found the following stored fault codes:
- – B220600 : Communication fault with camera control unit
- – U019B87 : Lost communication with Pre-Safe system
These codes indicated that the camera control unit’s software was outdated and could not synchronize with the radar data stream during certain conditions a known issue addressed by later Mercedes firmware updates.
Repair Procedure
- 1. Performed software update for the camera control unit (A40/11) using the latest Mercedes calibration file.
- 2. Cleared all fault codes and performed a sensor calibration to ensure synchronization between radar and camera data.
- 3. Checked communication logs on CAN and FlexRay buses to verify signal integrity.
- 4. Test-drove the vehicle for 25 km under varied traffic conditions to confirm fault resolution.
Result
After the software update and calibration, the “Pre Safe Functions Limited” message no longer appeared. The system resumed full functionality, and all control modules reported normal operation.
Key Lesson: Software compatibility plays a major role in modern ADAS performance. Regular camera and radar module updates are essential to prevent false warnings and maintain full Pre-Safe capability.
DIY vs Professional Repair
| Task | DIY Level | Recommendation |
|---|---|---|
| Cleaning radar and camera lenses | Easy | Safe for DIY maintenance |
| Checking battery voltage | Basic multimeter skill | DIY-friendly |
| Scanning and reading DTCs | Intermediate | Requires OBD or professional tool |
| Inspecting connectors behind bumper | Moderate | May require partial disassembly |
| Software updates or re-flashing control units | Not DIY | Dealer or Mercedes specialist required |
Preventive Maintenance Tips
- – Clean camera and radar areas regularly especially after heavy rain or snow.
- – Replace auxiliary batteries every 4–5 years to maintain voltage stability.
- – Avoid aftermarket radar or camera components; they may not align with OEM calibration data.
- – Perform ADAS calibration after any front-end repair or windshield replacement.
- – Update Pre-Safe and camera control unit firmware during routine servicing.
Technician Insight
“Many Pre-Safe warnings are not caused by physical defects but by software incompatibility between radar, camera, and braking modules. Always verify module firmware and calibration after repairs.”
Conclusion
The “Pre Safe Functions Limited” warning in Mercedes vehicles usually points to communication, calibration, or software-related issues between the radar and camera systems.
Through proper diagnosis, module updates, and calibration, technicians can restore full safety functionality without unnecessary part replacements.
For more case studies and repair insights on ADAS and radar system faults, explore:
Mercedes Driver Assistance System Faults: Radar, Blind Spot & Parktronic Fixes
Author Bio
Written by Mercedes Expert
With years of hands-on experience diagnosing and repairing Mercedes-Benz systems, he brings technical depth and practical case studies to help car owners, technicians, and enthusiasts troubleshoot complex automotive issues. His work focuses on clear repair guides, OEM-level procedures, and knowledge-sharing to empower both professionals and drivers.
Last update: November 2025
— Salim, Mercedes Expert
Independent specialist in Mercedes-Benz diagnostics, CAN Bus analysis, troubleshooting case studies, and EV systems.
