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Mercedes-Benz Pre-Safe Functions Limited

“PRESAFE Functions Limited” warning means the Mercedes-Benz vehicle may need the supplemental restraint system serviced. Presafe is part of the restraint system that tries to minimize the injuries to the driver and passenger in case of an accident. For example, pre-safe will tighten the seat belt when you get into an accident to protect you and minimize the injuries.

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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 on the road. 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 tighten upfront seat belts to firmly 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.

The purpose of all these activities is 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? 

Mercedes presafe

The functions that are a part 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. For a start, it constantly monitors drivers’ inputs and looks for any signs of so-called ‘panic braking.’

Mercedes presafe limited warning

This happens when the driver suddenly and sharply releases the throttle and slams on the brakes. Another critical factor is how the vehicle behaves on the road, with skidding and sliding being telltale signs of a potential accident. The Pre-Safe feature gains perception by 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 about the situation in front of them. To do so, they use a pair of forward-facing cameras within their windshields and a series of radars. By combining data from all these sources, the system will recognize any potential obstacles and take appropriate actions.      

Why does this warning message occur? 

mercedes presafe limited

Because of its complexity and safety-critical function, the Pre-Safe system requires all involved 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 deal with 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 that are crucial for the operation of the Pre-Safe Brake. 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 the operation of one or more radars. It is worth knowing that an unprofessional windshield replacement or front-end damage can also cause these issues.

Conclusion 

mercedes presafe limited

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.  

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