Active Bonnet Malfunction
Mercedes engineering goes beyond performance and style. Hidden beneath the bonnet are sophisticated safety systems designed to react in fractions of a second.
Built into the bonnet hinges is technology that can raise the bonnet instantly, creating a protective gap to help absorb the force of an impact if a pedestrian is struck. It is the kind of feature you may never see in action, but when it is needed, you want it working flawlessly. This is the Mercedes Active Bonnet system.
When the system failed on a Mercedes A-Class in Wigan, it needed the attention of trained professionals. At Standish Service Station, our technicians used their Mercedes expertise and a structured diagnostic approach to pinpoint the problem and put it right on the first attempt. No unnecessary parts, no trial-and-error, just a targeted repair to restore this important pedestrian protection feature to full operation.
Here’s how we identified the exact cause, verified the fault, and returned this Mercedes to the road, ready to protect in those critical moments. All without the dealership price tag.
Mercedes Active Bonnet: How It Helps Reduce Injury Risk
The Active Bonnet system is a Mercedes safety feature created to minimise injury risk in the event of a pedestrian collision. Built into the rear of each bonnet hinge are small pyrotechnic devices, known as squibs, which can lift the back edge of the bonnet by roughly 80 mm in milliseconds. This action creates a gap between the bonnet and the rigid engine parts below, allowing the bonnet to absorb impact energy more effectively.
The system is managed by the car’s Supplemental Restraint System (SRS) module, which works with dedicated sensors in the front bumper. When these sensors detect impact forces consistent with a pedestrian strike, they send the signal to deploy the bonnet lifters. Each hinge is fitted with its own lifter, and once activated, the bonnet remains raised until reset. While it can be pushed down to make the car roadworthy again, the pedestrian protection function is only fully restored once the bonnet lifters have been professionally reset or replaced after deployment.
Assessing the Active Bonnet Malfunction Alert (Fault Code: B2730)
When this Mercedes A-Class arrived at Standish Service Station in Wigan, the owner reported a dashboard message: “Active Bonnet Malfunction – see owner’s manual.” This is a safety warning that demands attention. If the system is inactive, the car’s pedestrian protection feature will not function as designed.
We linked our Mercedes dealer-level diagnostic tool to the vehicle and ran a complete electronic scan. This is the same advanced equipment used by official Mercedes dealerships, giving us unrestricted access to manufacturer data and guided diagnostic procedures. The scan produced a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. It appeared that the SRS control unit had identified an issue with the squib in the left-hand bonnet lifter.
A fault code tells us where to begin, but not the underlying cause. To get the full picture, our technicians followed a structured diagnostic routine — examining serial data, checking wiring continuity, inspecting connectors, and evaluating related control modules. This process allowed us to pinpoint the fault with certainty and confirm there were no other problems that could cause the same alert to appear again.
Tracing the Root Cause of the Active Bonnet Malfunction
With the fault code identified, we moved on to a detailed investigation. At Standish Service Station in Wigan, our approach combines hands-on Mercedes experience with dealer-level diagnostic equipment and access to official manufacturer data.
Our first step was to review Mercedes-Benz’s technical guidance for Fault Code B2730, which confirmed the issue was connected to the squib circuit in the left bonnet lifter. The aim was to determine the exact reason the SRS control unit had logged the fault, rather than simply replacing the lifter without confirmation.
We compared the serial data from both bonnet lifter circuits. The right-hand side produced normal readings, but the left-hand side showed no signal at all — a strong indication of an open circuit. This comparison is a key part of the process, as it uses the functioning side as a control to confirm that the fault is real and confined to one area.
Next, we carried out a close inspection of the wiring and connectors leading to the left bonnet lifter. We checked for corrosion, physical damage, and loose contacts that could interrupt the SRS system. Continuity and resistance tests showed the wiring was intact and performing within Mercedes-Benz specifications.
With the wiring and connectors ruled out, and serial data still confirming no continuity from the left lifter squib, we were able to confirm the lifter was the source of the fault. This thorough, step-by-step process ensured only the defective part was replaced, controlling costs while restoring the pedestrian protection system to full operational readiness.
Replacing the Faulty Component in the Active Bonnet System
Once we had confirmed the left bonnet lifter as the source of the fault, we contacted the customer to explain the findings and outline the repair procedure. With any safety-related system, it’s vital that the customer understands the work involved before we begin.
Following Mercedes-Benz’s safety procedures for pyrotechnic components, our first step was to disconnect the battery and wait for the SRS system to fully discharge. This ensured a safe working environment before removing the faulty left-hand bonnet lifter.
We installed the new lifter in strict accordance with the manufacturer’s torque specifications, guaranteeing both a secure fit and accurate alignment. After installation, our Mercedes dealer-level diagnostic tool was used to compare serial data from both bonnet lifter circuits. The readings matched perfectly, confirming the system was balanced and ready to operate correctly.
A full post-repair check was then carried out, which included clearing the stored fault code, verifying the system through serial data analysis, and ensuring no other warnings were active. Once all checks were complete, the car was returned to the customer with the pedestrian protection system restored to its intended condition.
Mercedes Active Bonnet Fault Fixed and Pedestrian Protection Reinstated
With the new left bonnet lifter installed, matching serial data confirmed, and all post-repair tests passed, the repair was complete. The Active Bonnet malfunction warning was cleared, and the system was restored to its intended working condition.
By targeting only the defective part, our team delivered an accurate, cost-effective repair. Using our Mercedes expertise, dealer-level diagnostics, and a methodical approach, we ensured the system met the manufacturer’s safety standards from the moment it left our workshop.
If your Mercedes displays an Active Bonnet malfunction message, it’s essential to have it checked promptly. This is not just a dashboard warning — it’s a signal that part of your pedestrian protection system isn’t ready to function when needed and needs addressing urgently.
Make Sure Your Mercedes Safety Systems Are Ready to Respond
If your Mercedes displays an Active Bonnet Malfunction warning, it’s not something to put off. This system plays a vital role in your car’s pedestrian protection technology, and if it isn’t working, the safety it’s designed to deliver simply won’t activate in an emergency. If you’ve noticed the warning, call the Mercedes experts at Standish Service Station on 01257 422899.
Our technicians in Wigan use a proven diagnostic process, combined with dealer-level equipment, to identify the fault accurately the first time. This gives you complete confidence that your repair meets the highest safety and quality standards.
Why Standish Service Station?
- Free courtesy car to keep you on the road
- 12-month parts and labour guarantee with every repair
- Dealer-level expertise without the main dealer price tag
- Specialist tools, technical data, and experience in Mercedes safety systems
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Don’t take risks with your Mercedes safety systems. Call us today on 01257 422899 to arrange a technical inspection and restore your vehicle to full working order.
