Mercedes-Benz Pre-Safe Functions Limited

PRE-SAFE®” is a safety feature developed by Mercedes-Benz to prepare a vehicle for an impending collision. It does this by automatically taking measures to ensure the safety of the vehicle’s occupants. The system uses various sensors throughout the car to detect certain driving situations that suggest an accident might be imminent.

Common Causes

The most common causes that trigger pre-safe functions are limited to Mercedes-Benz vehicles.

• The seat belt retractor is disconnected or faulty.
• The seat headrest was released, or the wires to it were damaged.
• Weak or dead 12-volt battery.
• Previous minor collision.
• A previous potential accident was detected but then was avoided.
• Software glitches: Occasionally, the system might experience a glitch affecting its functionality.
• Electrical issues: This message can lead to loose connections, corroded wires, or malfunctioning fuses related to the PRE-SAFE® system.
• Component failures: Parts like motors that adjust the seats or seatbelts could malfunction.

What is a Pre-Safe feature in a Mercedes-Benz?

Pre-safe is a commercial name for Mercedes’ safety system that helps prevent accidents and reduces their consequences if they do happen. This system constantly monitors what the driver is doing and how the vehicle behaves. In case there is a possibility of a crash or loss of control, it will perform these preventive actions:

• The sunroof and windows will automatically close, stopping large debris from entering the vehicle in case of a rollover. However, the windows will not roll up to prevent them from closing on passengers’ fingers.
• The brakes will raise the pressure inside the calipers, which increases the braking force and improves the stopping effect. This ensures the vehicle will slow down as much as possible, regardless of how the driver presses the brake pedal.
• While braking, the vehicle will firmly tie up the seat belts to brace the passenger against the seat. This safety system will also reposition the backrests in the upright position as an additional protective measure. These actions will ensure a more favorable body position and prevent excessive movement if the crash happens.
• The audio system will generate a strong buzzing sound, triggering a natural reflex that makes the ears less sensitive. This helps prevent hearing damage from loud noises present during a crash.
• Lastly, this system will automatically activate the hazard lights while braking. This should warn other drivers about the vehicle coming to a sudden stop in front of them.

All these activities aim to prepare the vehicle and its occupants for a potential impact. This scales down the risk of injury for the driver and passengers in an accident.

It is also essential to know that some vehicles have a Pre-Safe Brake, an additional safety feature. This system scans the area in front of the car and warns the driver about any oncoming obstacles. If the driver fails to react, it will apply the brakes to prevent the accident or lessen its consequences.

How does this safety system work?

The functions of the Pre-Safe feature rely on several other vehicle safety and drivetrain systems. This is because it uses data from these systems to get all the necessary information for its operation. It constantly monitors drivers’ inputs and looks for any signs of so-called ‘panic braking.’

This happens when the driver suddenly and sharply releases the throttle and slams the brakes. Another critical factor is how the vehicle behaves on the road, with skidding and sliding telltale signs of a potential accident. The Pre-Safe feature gains perception using data from the ABS/ESP system and its wheel speed sensors.

Furthermore, vehicles that have the Pre-Safe Brake option need to have a clear perspective of the situation in front of them. They use forward-facing cameras and a series of radars within their windshields. By combining data from all these sources, the system will recognize potential obstacles and take appropriate actions.

Why does this warning message occur?

Because of its complexity and safety-critical function, the Pre-Safe system requires all components to operate optimally. Even the slightest irregularity with any of them may affect the overall system performance and unreliable operation.

If that happens, the vehicle will warn the driver about the current condition by displaying a ‘Pre-Safe functions limited’ message. There is a range of probable reasons that may cause this, and these are the most likely ones:

• Control software that operates the Pre-Safe and its features can contain bugs inside its code, affecting its function. In most cases, the software cannot handle demanding real-life situations and inputs from different sensors that seem implausible. Still, Mercedes is constantly developing their drivers, and installing the latest software update may help in such cases.
• Any issue with the ABS and ESP systems will, in most cases, affect Pre-Safe features and limit their function. Among potential topics, faulty wheel speed sensors and malfunctioning ESP control units are the most common. These failures will usually affect the operation of the ABS/ESP system, although their impact may be limited to Pre-Safe only. Tracking the cause of the problem using a diagnostic tool and replacing the faulty components will solve the issue.
• Driving in harsh conditions may cause issues with sensors crucial for the Pre-Safe Brake operation. Things like snow or heavy downpours may affect the function of forward-facing cameras, as they obstruct their fields of view. On the other hand, leaves and other road debris may interfere with operating one or more radars. It is worth knowing that an unprofessional windshield replacement or front-end damage can also cause these issues.

Conclusion

As we have explained in this article, the Mercedes Pre-Safe function is a feature that improves overall safety. In case of a likely crash, it conducts several actions to reduce its impact on the passengers inside the vehicle. These features rely on other systems and hardware, such as SRS / ABS/ESP or forward-facing cameras and radars. As a result, any failures in these systems will also affect how the Pre-Safe and its functions perform.

To diagnose a Pre-safe limited warning message, follow this guide and read the fault codes from the Airbag (SRS) module.

The YOUCANIC Full System Scanner can read and clear fault codes through every vehicle’s control module.

We hope you find the Mercedes-Benz Pre-Safe Functions Limited guide helpful. Check these troubleshooting and repair guides for more help on your Mercedes-Benz.