Active Bonnet Malfunction
Mercedes put more than just performance under the bonnet. They integrate advanced safety systems designed to react in an instant when it matters most.
Inside the bonnet hinges is a mechanism that can lift the bonnet within milliseconds, creating a protective gap to reduce the severity of an impact if a pedestrian is hit. It is technology you hope will never be needed, but when it is, you want it to work without fault. This is the Mercedes Active Bonnet system.
When a Mercedes A-Class in Cardiff developed a fault, it required the expertise of trained professionals to put right. At Ystrad Service Centre, our experienced technicians combined Mercedes-specific knowledge with a step-by-step diagnostic process to identify and fix the issue first time. No trial-and-error, no unnecessary replacements, just an accurate repair that restored this critical pedestrian protection feature to full function.
Here’s how we located the fault, confirmed the cause, and returned this Mercedes to the road ready to protect in an emergency — all without the dealer price tag.
Understanding the Mercedes Active Bonnet
The Active Bonnet system is a safety innovation from Mercedes designed to help reduce the severity of injuries in a pedestrian collision. Located at the back of each bonnet hinge are compact pyrotechnic devices, called squibs, which can raise the rear edge of the bonnet by around 80 mm in a split second. This lift creates a gap between the bonnet and the solid engine components underneath, allowing the bonnet to deform more gradually and absorb more of the impact energy.
Control of the system comes from the car’s Supplemental Restraint System (SRS) module, which works alongside dedicated impact sensors in the front bumper. When these sensors detect collision forces that match a pedestrian strike, they signal the bonnet lifters to deploy. Each hinge has its own lifter, and once activated, the bonnet stays in the raised position until it is reset. Lowering the bonnet makes the car usable again, but the pedestrian safety system regains full readiness only once the lifters have been properly reset or changed after deployment.
Initial Diagnosis of the Active Bonnet Malfunction (Fault Code: B2730)
When this Mercedes A-Class was brought to Ystrad Service Centre in Cardiff, the driver explained that a dashboard message had appeared: “Active Bonnet Malfunction – see owner’s manual.” This warning should never be ignored. If the system is disabled, the pedestrian protection mechanism will not respond in an emergency.
We connected our Mercedes dealer-level diagnostic equipment and performed a full vehicle scan. This is the same high-grade technology used by Mercedes main dealers, giving us complete access to official manufacturer data and guided test steps. The scan confirmed a stored code: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In simple terms, the SRS control unit had detected a problem with the squib in the left bonnet lifter.
While the fault code narrowed down the area to investigate, it didn’t identify the exact cause. To find that, our technicians carried out a detailed, step-by-step process: analysing serial data, testing wiring integrity, inspecting connectors for signs of wear or corrosion, and checking linked control modules. This ensured we could confirm the root cause with confidence and rule out any other faults that might cause the system to log the same warning in future.
Uncovering the Cause of the Active Bonnet System Fault
After confirming the fault code, our next task was to investigate thoroughly. At Ystrad Service Centre in Cardiff, we follow a clear, structured process that combines our Mercedes knowledge with dealer-level diagnostic tools and official manufacturer data.
We began by consulting Mercedes-Benz’s technical information for Fault Code B2730, which linked the issue to the squib circuit in the left bonnet lifter. Our priority was to discover why the SRS control unit had logged the fault, rather than assuming the lifter itself was the problem.
We then compared serial data from both bonnet lifter circuits. The right-hand circuit showed normal readings, while the left-hand side produced no signal — an indication of an open circuit. This comparison is essential, as the working side provides a reference to confirm the fault is genuine and confined to one location.
Next, we examined the wiring and connectors supplying the left bonnet lifter, looking for corrosion, physical damage, or loose contacts that could disrupt the SRS system. Continuity and resistance tests showed the wiring was in good condition and met Mercedes-Benz’s specifications.
With wiring and connectors confirmed as fault-free, and serial data still indicating no continuity from the left lifter squib, we determined the lifter itself was at fault. This step-by-step, evidence-led approach ensured that only the failed component was replaced, helping to manage costs while fully restoring the pedestrian protection system.
Completing the Active Bonnet System Repair
With the fault confirmed in the left bonnet lifter, we updated the customer on our findings and explained the work required to restore the system. Transparency is essential for safety-critical repairs, so we made sure they were fully informed before starting.
We began by following Mercedes-Benz’s official safety guidance for handling pyrotechnic components. This involved disconnecting the battery and waiting for the SRS system to discharge completely, ensuring it was safe to remove the faulty left-hand bonnet lifter.
The new lifter was installed in line with Mercedes-Benz’s torque specifications to ensure an accurate fit and secure mounting. Once fitted, we connected our Mercedes dealer-level diagnostic tool and compared the serial data from the left and right bonnet lifter circuits. Identical readings confirmed both were functioning exactly as they should — a necessary step to ensure the control unit wouldn’t disable the system due to an imbalance.
We then carried out a full post-repair inspection, clearing the stored fault code, confirming system performance through serial data checks, and making sure no other warnings were present. With all tests passed, we returned the Mercedes to its owner, knowing the pedestrian protection system was fully restored and ready for action.
Error Code B2730 Cleared – Mercedes A-Class Restored
With the faulty bonnet lifter replaced, serial data readings matched on both sides, and every verification test completed, the repair was signed off. The Active Bonnet malfunction warning was removed, and the system was fully restored to manufacturer specifications.
By focusing solely on the confirmed faulty component, we provided a cost-conscious repair. Our technicians combined Mercedes know-how, dealer-level tools, and a step-by-step approach to deliver a first-time fix that met strict safety requirements.
If your car shows an Active Bonnet malfunction warning or logs Fault Code B2730, don’t ignore it. This is a vital part of your vehicle’s pedestrian protection system, and without it working correctly, you’re missing an important layer of safety.
Keep Your Mercedes Active Bonnet System Working as Intended
If your Mercedes has logged Fault Code B2730 or is showing an Active Bonnet Malfunction warning, it’s a sign you should act quickly. This system is a key element of your car’s pedestrian protection technology, and without it functioning, the safety feature it’s designed to provide won’t respond when it’s most needed. If you’ve spotted the warning, call the Mercedes specialists at Ystrad Service Centre on 01443 540460.
At our Ystrad Mynach workshop, we follow a clear, methodical diagnostic process, supported by dealer-level equipment, to pinpoint the problem accurately the first time. This means you can drive away knowing your safety systems are working exactly as intended.
Why choose Ystrad Service Centre?
- Free courtesy car to keep you moving while we work
- 12-month parts and labour guarantee on every repair
- Dealer-level technical knowledge without dealer prices
- Specialist equipment, technical data, and training in Mercedes safety systems
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Don’t take chances with your Mercedes safety systems. Call Ystrad Service Centre today on 01443 540460 to arrange a full inspection and return your vehicle to peak condition.
