Active Bonnet Malfunction: Solved! (Step by Step)

active bonnet Malfunction

How do I reset the Mercedes active bonnet malfunction? Whenever a new error message shows up, you always feel a moment of nervousness and confusion about what it means and what it’s going to take to make it go away.

If you’ve been warned that you have an “active bonnet malfunction”, then you’ve come to the right place to sort it out.

The “active bonnet malfunction” error message is unique to Mercedes-Benz vehicles with the active bonnet feature installed in the car. When you see the error, it typically means that the system has been tripped due to a fault or the actuators are not properly seated.

Here are a few steps on how to solve active bonnet malfunction including:

Step 1: Check For Damage

Step 2: Identify Whether The Active Bonnet Has Triggered

Step 3: Push Down On The Bonnet

Step 4: Listen For Re-Engagement

Step 5: Test The Bonnet

What Is The Active Bonnet System?

To understand what has gone wrong that has caused this message to appear, then we need to first figure out what it is actually referring to.

In other vehicles, you might get some kind of generic error regarding your bonnet, or you might see a warning light turn on, but the “active bonnet malfunction” in a Mercedes is referring to a specific system that these cars have.

The active bonnet is a pedestrian protection feature that is designed to reduce the risk of injury to pedestrians in case of a sudden impact. It is present on almost all Mercedes-Benz cars made from 2010 onwards and is standard in their newest models.

How To Fix Active Bonnet Malfunction

Fortunately, the steps to solving this problem are usually very simple!

Step 1: Check For Damage

First, make sure that you are in a safe place, the engine has been switched off and there has not been an impact that you were not aware of.

Check the bonnet for signs of damage, and make sure that it is cool and there is no smoke or other signs of danger before continuing.

Step 2: Identify Whether The Active Bonnet Has Triggered

You will be able to see whether or not the active bonnet has been triggered just by looking at it.

It will be pretty noticeable, as the edge of the bonnet closest to the windshield will have raised up by around 50-70mm, almost like it has been opened from the wrong side.

Step 3: Push Down On The Bonnet

If the active bonnet has been engaged, then you simply press your palms down firmly on both sides of the bonnet, where the hinges are, pushing it back into place.

If the bonnet has not lifted up in any way but you’re still getting the error message, then you might be dealing with a sensor problem or damage to one of the components. This usually means that you will need to take the car to a professional mechanic for a proper diagnosis.

Step 4: Listen For Re-Engagement

You will know that the spring actuators have been reset if they depressurize and stay closed. You might hear a faint hissing sound while this is happening.

Step 5: Test The Bonnet

Now, try and lift the bonnet where the hinges are. If it does not move, then the system should be reset and everything should be back to normal. If not, repeat steps 3 and 4 until it is correctly engaged.

Then, simply have a look at the screen to see whether or not the message has disappeared.

How The Active Bonnet Works

So that we can identify what might malfunction in this active bonnet system, we need to know a little about how it works.

It’s made up of four components:

  • Acceleration sensors. To identify that an impact has occurred.
  • Power electronics. That register information from the sensors and activate the system.
  • Spring actuators. When activated, these lift the bonnet.
  • Bonnet hinges. These are designed to lift with the actuators and can be reset to their original position.

If the front of the car strikes something, like a pedestrian, then the rear end of the bonnet – closest to the windshield – will lift up by between 50 and 70 millimeters, depending on the specific class of the vehicle.

The three acceleration sensors, two in the bumper and one on the crossmember, identify when the bonnet is struck and send information directly to an electronic control unit.

This will activate two solenoids in special bonnet hinges, extending the springs and causing the bonnet to lift. All of this happens in less than a second.

This system makes the deformation area of the bonnet larger, allowing it to absorb more of the impact and reducing the amount of damage that might be caused to a pedestrian that may have hit the bonnet.

What Does An Active Bonnet Malfunction Mean?

Now that we know what the active bonnet is, and how it works, we can start to understand what it might mean when your vehicle is telling you that it has malfunctioned.

There are three main reasons why you might see this message:

  • The active bonnet has been tripped. Either due to an impact or some kind of issue with the sensors that means they are incorrectly registering that an impact has occurred.
  • The actuators are not seated properly. This can mean that the springs activate and the bonnet lifts without the sensors registering any impact at all.
  • There has been some damage to a component of the bonnet. Either the actuators, the sensors, the hood latch, or the SRS module might not be working properly, and show up as an active bonnet malfunction – or they can trigger the active bonnet improperly.

Re-Seating The Actuators

If the message keeps appearing, the springs are not engaging properly, or the active bonnet system is triggered again without any kind of impact, then you might need to re-seat the actuators under the hood manually.

To do this, lift the bonnet up in the middle until the lifters can no longer move. Then, push on the lifter lids. You should notice some resistance. Then, release the bonnet and make sure that the lifters are sitting on their housings. If they are, then you should be able to push the bonnet back down and it should reset normally.

If the actuators are not re-seating themselves properly, then you may need to visit a mechanic to carry out the re-seating for you, or they might be damaged and need replacing completely.

What Is The Problem With Bonnet Warning Light On Mercedes?

The other major bonnet error that you might see in your Mercedes, or any other car really, is when the bonnet warning light comes on – but what does it mean?

The bonnet warning light is designed to tell you that the bonnet is not securely closed, which is a safety risk. This might happen because you did not fully shut the hood the last time that you had it open, or it could mean that something like the hood latch is damaged or not working properly.

How Do You Check A Bonnet?

If the bonnet warning light comes on, then you will want to carry out some basic checks to see if any components are damaged that might be stopping the bonnet from staying closed.

The first thing you need to do is pop the hood and, if you’re used to other kinds of cars, you might find yourself wondering: where is the bonnet button on a Mercedes?

These cars don’t have a bonnet button. What you’re looking for is the hood release lever, which is usually on the driver’s side: underneath the dashboard, and just above the parking brake. When you pull on this lever, you should hear the bonnet pop open – although it might take a couple of tries.

Once you’ve pulled on this lever, you will need to go to the front of the car, get your fingers underneath the hood in the middle, and find the hood latch release.

Pull this towards you, and the bonnet should open freely. To keep it in place while you have a look around, you will need to prop it open with the bonnet stay.

Can I Drive With A Broken Hood Latch?

Your bonnet might open when it’s not supposed to, or it might not open properly at all, and this often means that the hood latch is damaged in some way.

Although this is not part of the active bonnet system, it can even cause the hood to move unexpectedly and trigger the mechanism – or the car might just register this as an active bonnet malfunction anyway.

While you might be able to drive with a broken hood latch, it can be incredibly dangerous. Even if the bonnet seems to be staying in place, this can change suddenly while you are moving if the damage worsens, or if the latch fails completely – and you could end up with a completely open hood and a severely obstructed view.

It’s less likely to cause problems if your hood latch is not allowing the bonnet to open, but it’s still a significant risk.

Where possible, you shouldn’t even drive your vehicle to the mechanic if the hood latch is not functioning properly.

Read also >> Drive-Start Control Malfunction (Step by Step Guide!)

Read also >> Drivetrain Malfunction: How To Fix? (Step by Step)

What Happens If You Drive With Your Bonnet Open?

If you really need to get your car in for repairs and the hood just won’t stay closed, you might be tempted to drive it anyway, but this is a very bad idea.

The bonnet will be caught by the air as you move, causing it to lift up in front of the windshield and making it almost impossible for you to see where you are going. It can even strike the windshield behind it, causing it to shatter and potentially harming you in the process.

If the bonnet opens while you are driving, slow down and find a place to safely pull over. Then, try to re-engage the hood latch, testing it carefully to make sure that it is functioning normally.

If you can’t be sure that it will stay closed when you start driving again, then you should call a mechanic to come and fix the problem before you drive off.



So, what is an active bonnet malfunction? This error message is specific to Mercedes-Benz vehicles that have the active bonnet system installed – and it usually means that this system has either been triggered or is not working properly.

Usually, you simply need to reset the active bonnet by pressing it back into place, but you may need to repair or replace individual components within the system or re-seat the actuators into position so that they are working properly.

The other common error you might see is a bonnet warning light, which simply means that the bonnet isn’t properly closed.


Steve P.

Steve is an automotive technician, technical writer, and Managing Editor. He has held a lifelong passion for cars, with a particular interest in cars like the Buick Riviera. Steve is based in Boise, Idaho.

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