Active Bonnet Malfunction
Mercedes engineering is about more than performance — it also hides advanced safety systems designed to respond in fractions of a second.
Built into the bonnet hinges of many Mercedes models is a mechanism that can raise the bonnet instantly, creating a buffer to absorb energy if a pedestrian steps into your path. You hope you’ll never see this feature in action, but if the situation arises, you want it working flawlessly. This is the Mercedes Active Bonnet system.
When the system failed on an A-Class in Swansea, it needed the attention of trained professionals. At Garden City Garage, our technicians combined in-depth Mercedes know-how with a clear, step-by-step diagnostic process to pinpoint and fix the problem first time. No unnecessary parts, no guesswork — just an accurate repair that restored this important pedestrian protection feature to full working order.
Here’s how we identified the fault, confirmed it beyond doubt, and returned this Mercedes ready for the moments when safety matters most — without the dealership price tag.
Explaining The Role of the Mercedes Active Bonnet
The Active Bonnet system is a Mercedes safety innovation designed to help reduce the severity of injury if a pedestrian is struck. Positioned at the rear of each bonnet hinge are small pyrotechnic devices, known as squibs, which can raise the back edge of the bonnet by around 80 mm in just milliseconds. This rapid movement creates a gap between the bonnet and the hard engine components underneath, allowing the bonnet to absorb energy more effectively during an impact.
The system is controlled by the vehicle’s Supplemental Restraint System (SRS) unit, which works alongside specialist impact sensors in the front bumper. When these sensors detect a collision pattern consistent with a pedestrian impact, they signal the bonnet lifters to deploy. Each hinge has its own lifter, and once deployed, the bonnet stays raised until it is reset. While it can be pushed back down to make the car driveable, the pedestrian protection capability will only be fully restored once the bonnet lifters have been correctly reset or replaced following deployment.
First Look at the Active Bonnet Malfunction Warning (Fault Code: B2730)
When this Mercedes A-Class arrived at Garden City Garage in Swansea, the driver reported a dashboard warning: “Active Bonnet Malfunction – see owner’s manual.” This is not the kind of message to ignore. If the system is inactive, the vehicle’s pedestrian protection feature won’t respond in the moments it’s designed to save lives.
We connected our Mercedes dealer-level diagnostic equipment and carried out a complete vehicle scan. This is the same advanced equipment used in main Mercedes dealerships, giving us full access to official technical data and guided testing routines. The scan revealed a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In simple terms, the SRS control unit had detected an issue with the squib inside the left-hand bonnet lifter.
A fault code alone doesn’t tell the whole story, it’s simply a starting point. To uncover the real cause, our technicians followed a structured diagnostic process, reviewing serial data, testing wiring integrity, inspecting connectors, and assessing any related control modules. This approach ensured we identified the exact source of the fault and ruled out any other issues that could cause the warning to return.
Digging Deeper to Find the Active Bonnet Malfunction Cause
With the initial fault code identified, our next step was to investigate in detail. At Garden City Garage in Swansea, this means combining our Mercedes expertise with dealer-level tools and official manufacturer data to guide every stage of the process.
We began by consulting Mercedes-Benz’s technical documentation for Fault Code B2730, which confirmed the issue was linked to the squib circuit in the left bonnet lifter. The aim was to uncover why the SRS control unit had registered the fault — not simply assume the lifter itself was defective.
To build a clear picture, we compared the serial data from both bonnet lifter circuits. The right-hand side produced normal readings, while the left-hand side showed no measurable signal, indicating an open circuit. This comparison is an important step, as it allows us to use the functioning side as a reference point and verify that the fault is isolated.
We then inspected the wiring and connectors supplying the left bonnet lifter, checking carefully for corrosion, physical wear, or loose connections. Even a minor issue here can cause the SRS system to log a fault. Continuity and resistance tests confirmed that the wiring was intact and operating within Mercedes-Benz’s specifications.
With wiring and connectors ruled out, and serial data consistently showing no continuity from the left lifter squib, we could be certain that the lifter itself was at fault. By following this evidence-based process, we ensured that only the defective component was replaced, keeping costs under control while restoring the pedestrian protection system to full readiness.
How We Repaired the Active Bonnet Malfunction
With the fault traced to the left bonnet lifter, we contacted the customer to explain our findings and the steps needed for repair. Clarity is essential when dealing with a safety-critical system, so we outlined the process in full before starting work.
Following Mercedes-Benz’s official safety procedures for handling pyrotechnic components, we began by disconnecting the battery and allowing the SRS system to fully discharge. This ensured the system was completely safe before removing the faulty left-hand bonnet lifter.
The new lifter was fitted in line with the manufacturer’s torque specifications to guarantee a secure and accurate installation. Once in place, we used our Mercedes dealer-level diagnostic tool to compare the serial data from both bonnet lifter circuits. Matching readings confirmed that both sides were operating identically — a critical check to prevent the control unit from detecting an imbalance and disabling the system.
We then carried out a complete post-repair inspection, clearing the stored fault code, verifying the Active Bonnet system through serial data checks, and ensuring no related warnings were present. Only when every test passed did we return the car to the customer, confident the pedestrian protection feature was fully restored and ready to respond if needed.
Repair Complete – Active Bonnet System Ready to Protect
With the new bonnet lifter installed, serial data verified, and all checks passed, the repair was complete. The Active Bonnet Malfunction warning was cleared, and the system was once again ready to perform its role in an emergency.
By addressing the fault precisely and replacing only the defective component, our technicians ensured the repair was both accurate and cost-effective. Using dealer-level technical data, specialist tools, and a structured diagnostic approach, we delivered a first-time fix that met Mercedes-Benz safety standards.
If your Mercedes displays an Active Bonnet Malfunction warning, it’s important to act quickly. The system is a key part of the vehicle’s pedestrian protection design, and without it functioning, you lose an important safety feature.
Ensure Your Mercedes Safety Features Are Working at Their Best
If your Mercedes shows an Active Bonnet Malfunction warning, it’s a sign that demands immediate attention. This system is a key part of your car’s pedestrian protection technology, and without it working, the safety response it’s built to deliver will not activate in a critical moment. If you think there’s an issue, contact the Mercedes specialists at Garden City Garage on 01639 701604.
At our Seven Sisters workshop, we pair in-depth Mercedes knowledge with a systematic diagnostic routine and dealer-level tooling to identify the cause of faults with precision. This approach ensures the job is done right the first time, giving you total peace of mind when you drive away.
Why Garden City Garage?
- All repairs covered by a 12-month parts and labour guarantee
- Main-dealer level skill and capability at independent garage prices
- Advanced equipment, technical data, and expertise in Mercedes safety systems
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Don’t compromise on safety. Call Garden City Garage today on 01639 701604 to arrange a full technical evaluation and return your Mercedes to peak condition.
