Active Bonnet Malfunction
Mercedes vehicles are built with far more than engine power under the bonnet. They also hide advanced safety systems designed to react in an instant.
Within the bonnet hinges is a mechanism that can raise the bonnet in milliseconds, creating a protective gap to help absorb impact if a pedestrian steps into your path. It’s the kind of technology you hope will never be used, but if it’s needed, you want it performing perfectly. This is the Mercedes Active Bonnet system.
When the system failed on a Mercedes A-Class in Sittingbourne, it needed the attention of trained professionals. At Gents Garage, our team drew on specialist Mercedes knowledge and a methodical diagnostic process to find and fix the fault the first time. No trial-and-error and no unnecessary parts, just a precise repair that returned this safety feature to full working order.
Here’s how we traced the fault, confirmed the cause beyond doubt, and restored this Mercedes to full readiness, all without the dealership price tag.
Inside the Mercedes Active Bonnet System and How It Operates
The Active Bonnet system is one of Mercedes’ advanced safety features, designed to lower the risk of serious injury if a pedestrian is struck. Located at the rear of each bonnet hinge are compact pyrotechnic devices, called squibs, which can raise the bonnet’s back edge by approximately 80 mm in a fraction of a second. This sudden lift creates a protective gap between the bonnet and the solid engine components beneath, helping the bonnet to deform more gradually and absorb impact energy.
Control of the system comes from the vehicle’s Supplemental Restraint System (SRS) module, working with specially positioned sensors in the front bumper. If these sensors detect collision forces matching a pedestrian impact, they send a signal to activate the bonnet lifters. Each hinge has its own lifter, and once triggered, the bonnet remains in its raised position until it is reset. While it can be pushed back down to make the car driveable, the pedestrian protection capability will not be restored until seen by a professional.
Our First Steps in Diagnosing the Active Bonnet Malfunction (Fault Code: B2730)
When this Mercedes A-Class was brought to Gents Garage in Sittingbourne, the owner mentioned a dashboard alert reading “Active Bonnet Malfunction – see owner’s manual.” This isn’t a warning to take lightly. If the system isn’t operational, the pedestrian protection element won’t deploy when it’s needed most.
We connected our Mercedes dealer-level diagnostic tool and performed a full system scan. This is the same level of equipment used in Mercedes main dealerships, giving us access to manufacturer-grade technical data and guided test sequences. The scan flagged a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. Put simply, the SRS control unit had registered a problem with the squib in the left-hand bonnet lifter.
While a fault code points us in the right direction, it doesn’t confirm the root cause. To find that, our technicians used a clear, methodical process: checking serial data, testing the wiring harness, inspecting connectors for signs of wear or corrosion, and reviewing any linked control modules. This thorough approach meant we could confirm the true cause and ensure there were no other issues that might trigger the same warning in future.
Pinpointing the Problem Behind the Active Bonnet Fault
Once the fault code had been logged, the next step was to investigate fully. At Gents Garage in Sittingbourne, that means using our Mercedes-specific knowledge, supported by dealer-level diagnostic tools and official manufacturer guidance, to work through a structured testing routine.
We began by checking Mercedes-Benz’s technical notes for Fault Code B2730, which confirmed the fault related to the squib circuit in the left bonnet lifter. The key was to determine exactly why the SRS control unit had flagged the problem, rather than assuming the lifter itself was defective.
Using the serial data from the bonnet lifter circuits, we compared the readings from each side. The right-hand circuit returned normal values, while the left-hand side showed no output, indicating an open circuit. This side-by-side analysis is important because the working side provides a baseline, helping confirm that the fault is both genuine and confined to one side.
Next, we examined the wiring and connectors to the left bonnet lifter, looking for any signs of corrosion, damage, or looseness that could disrupt the SRS system. We then ran continuity and resistance checks, which showed the wiring was sound and within Mercedes-Benz’s specification.
With the wiring and connectors confirmed as fault-free, and serial data still showing no continuity for the left lifter squib, we concluded the lifter itself was faulty. This methodical, evidence-driven process meant we could replace just the failed part, avoiding unnecessary costs and returning the pedestrian protection system to its correct working state.
Restoring the Active Bonnet System to Full Working Order
With the cause of the fault confirmed as the left bonnet lifter, we spoke with the customer to explain our findings and the exact steps required to carry out the repair. Safety-related systems require full transparency, so we made sure they understood the process before giving us the go-ahead.
We began by following Mercedes-Benz’s official safety guidelines for working with pyrotechnic components. This included disconnecting the battery and allowing the SRS system to fully discharge, ensuring the work area was safe before the removal of the defective left-hand bonnet lifter.
The replacement lifter was installed according to the manufacturer’s torque settings, ensuring a precise fit and secure mounting. Once fitted, we used our Mercedes dealer-level diagnostic tool to compare serial data from the left and right bonnet lifter circuits. Identical readings confirmed both sides were performing exactly as intended — an essential step, as any imbalance could cause the control unit to deactivate the system.
Finally, we ran a comprehensive post-repair inspection. This involved clearing the stored fault code, rechecking serial data to verify the system’s performance, and confirming no other warnings were present. With every test passed, the Mercedes was returned to the customer with its pedestrian protection system fully operational and ready for use.
Active Bonnet System Fully Operational Once Again
With the replacement bonnet lifter fitted, serial data matched on both sides, and every post-repair check completed, the job was finished. The Active Bonnet Malfunction warning was cleared, confirming the system was back to full operational status.
By replacing only the confirmed faulty component, we ensured the repair was both accurate and cost-conscious. Our technicians combined Mercedes-specific expertise, dealer-level equipment, and an evidence-based process to deliver a first-time fix that met the manufacturer’s safety requirements.
If your dashboard shows an Active Bonnet Malfunction warning, don’t delay in having it checked. This system is an integral part of your car’s pedestrian protection measures, and leaving it inactive removes an important safety safeguard.
Keep Your Mercedes Active Bonnet System in Top Condition
If your Mercedes is showing an Active Bonnet Malfunction warning, it’s not an alert to ignore. This feature is central to your car’s pedestrian protection system, and if it’s not active, the safety it’s designed to deliver won’t be available in an emergency. If you notice the warning, contact the Mercedes specialists at Gents Garage on 01795 425999.
At our Sittingbourne workshop, we combine specialist Mercedes knowledge with a clear, step-by-step diagnostic process and dealer-level tooling. This ensures we find the precise cause of the fault first time, restoring your confidence in your vehicle’s safety systems.
Why choose Gents Garage?
- 12-month or 10,000-mile parts and labour warranty on all repairs
- Dealer-level technical capability without the main dealer cost
- Specialist equipment, information, and expertise in Mercedes safety technology
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Don’t take risks with your Mercedes safety systems. Call Gents Garage today on 01795 425999 to arrange a full inspection and get your car back to full working order.
