Active Bonnet Malfunction
Mercedes safety engineering goes far beyond airbags and brakes — some of it is hidden in places you wouldn’t expect.
Built into the bonnet hinges is a precision mechanism that can raise the bonnet in an instant, helping to cushion a pedestrian impact. It’s technology you might never see in action, but if it’s needed, it has to work exactly as intended. This is the Mercedes Active Bonnet system.
When a 2021 A-Class in Wymondham developed a fault in this system, it needed the expertise of CSN Autos. As multi-award-winning Mercedes specialists, our team combined proven diagnostic techniques with dealer-level equipment to find the fault and fix it right first time. No unnecessary part swaps, no trial-and-error — just accurate diagnosis and a complete repair to restore the vehicle’s pedestrian protection.
Here’s how we located the exact issue, verified our findings, and returned the car to the road with its safety systems fully reactivated — and all at a competitive price.
How Mercedes’ Active Bonnet Technology Works to Reduce Impact Risk
The Active Bonnet system is a Mercedes safety innovation designed to help lower the chance of severe injury if a pedestrian is hit. Positioned at the back of each bonnet hinge are small pyrotechnic devices, known as squibs, which can raise the rear edge of the bonnet by roughly 80 mm in a split second. This creates a cushioning gap between the bonnet and the hard engine components underneath, allowing the bonnet to absorb energy more gradually during a collision.
The system is operated by the car’s Supplemental Restraint System (SRS) control unit, working alongside purpose-built impact sensors within the front bumper. When these sensors register a collision pattern matching that of a pedestrian strike, they send the signal for the bonnet lifters to deploy. Each hinge has its own lifter, and once activated, the bonnet stays raised until it’s reset. While it can be lowered manually to make the vehicle driveable, the pedestrian protection system won’t be active again until the cause is correctly fixed.
Looking Into The Active Bonnet Malfunction on Arrival (Fault Code: B2730)
When this Mercedes A-Class was brought to CSN Autos in Wymondham, the owner mentioned a dashboard alert: “Active Bonnet Malfunction – see owner’s manual.” This is not the type of message to ignore. If the system isn’t functioning, that extra level of pedestrian protection won’t be available when it matters most.
We hooked up our Mercedes dealer-level diagnostic tool and performed a full vehicle scan. This is the same advanced technology used by main Mercedes dealerships, giving us complete access to manufacturer data and guided testing procedures. The scan identified a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In simple terms, the SRS control unit had found an issue with the squib in the left-hand bonnet lifter.
A fault code alone doesn’t give the complete picture — it’s only the starting point. To find the exact cause, our technicians followed a clear, methodical diagnostic routine, analysing serial data, checking the wiring, inspecting connectors, and assessing any related control modules. This ensured we located the precise source of the fault and ruled out any other problems that could cause the system to trigger the same warning again.
Getting to the Bottom of the Active Bonnet Malfunction
Once the initial scan was complete, we began a detailed investigation. At CSN Autos in Wymondham, this involves following a structured diagnostic routine, using our specialist Mercedes knowledge supported by dealer-level equipment and manufacturer technical data.
We started by checking Mercedes-Benz’s official guidance for Fault Code B2730, which confirmed it relates to a problem in the squib circuit for the left bonnet lifter. This gave us a clear objective: find the reason behind the fault code, rather than assuming the lifter itself was the issue.
Next, 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 output — a strong sign of an open circuit. This comparison process is critical, as the functioning side acts as a control, confirming the fault is real and confined to one side.
We then carried out a close inspection of the wiring harness and connectors feeding the left bonnet lifter, checking for corrosion, physical damage, or loose terminals. Even minor connection faults can cause a failure in the SRS system. Continuity and resistance tests confirmed the wiring was intact and operating within Mercedes-Benz’s specifications.
With the wiring and connectors in good condition, and serial data consistently showing no continuity on the left lifter squib, we concluded with certainty that the lifter itself was faulty. By following this clear, evidence-based process, we ensured we replaced only the component that had failed, avoiding unnecessary costs and returning this key pedestrian protection system to full function.
Fixing the Active Bonnet Malfunction and Restoring Safety Standards
Once we had identified the cause, we spoke with the customer to outline our findings and the steps needed to put things right. Transparency is vital when repairing a system as safety-critical as the Active Bonnet. With the customer’s consent, we followed Mercedes-Benz’s strict procedures for working on pyrotechnic devices, starting with disconnecting the battery and allowing the SRS system to discharge completely before removing the faulty left-hand bonnet lifter.
The replacement lifter was installed according to Mercedes-Benz torque settings, ensuring a secure fit and correct alignment. We then compared the readings from both bonnet lifter circuits to confirm they matched exactly. This step is essential — without balanced readings, the control unit could log another fault and disable the system.
To complete the job, we performed a full post-repair check. This involved clearing the stored fault code, using serial data to confirm the Active Bonnet system was back in proper working order, and checking for any other related issues. Once all safety checks were passed, the Mercedes was ready to return to the road with its pedestrian protection fully restored.
Active Bonnet Malfunction Fixed and System Fully Functional
Following the repair, this Mercedes A-Class left our Wymondham workshop with the Active Bonnet Malfunction warning cleared and the system working exactly as designed. As an integral part of the car’s pedestrian protection system, the active bonnet was once again ready to deploy instantly if needed.
Our in-depth Mercedes experience, paired with a clear, evidence-led diagnostic process, meant we could identify the precise cause of the fault and replace only the failed component. Supported by dealer-level technical information and the right specialist tools, we delivered a first-time fix that fully met Mercedes-Benz safety standards.
For the owner, it meant far more than simply removing a warning message. It gave them the confidence that their Mercedes’ safety systems were fully restored and ready to react in the split seconds that matter most, along with peace of mind that the work had been completed to the highest professional standard.
Professional Active Bonnet Repairs for Complete Peace of Mind
If your Mercedes displays an Active Bonnet Malfunction warning, it’s a sign that your pedestrian protection system isn’t ready to respond when it matters most. Leaving it unresolved could mean losing a vital safety feature. If you spot a warning light or suspect a fault, call the Mercedes specialists at CSN Autos in Wymondham on 01953 425983.
Our multi-award-winning team uses a structured, methodical diagnostic process, backed by dealer-level tools, to identify the issue accurately the first time and ensure every repair meets Mercedes-Benz safety standards.
Other benefits of choosing CSN Autos include:
- Free courtesy car while we work on your vehicle
- 12-month parts and labour guarantee
- Dealer-level expertise without dealer prices
- Specialist knowledge in Mercedes safety systems
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Don’t wait until it’s too late. Call 01953 425983 today to book a professional assessment and get your Mercedes back to full working order.
