Active Bonnet Malfunction
Mercedes engineering hides more than just performance under the bonnet — it also houses safety technology designed to act in milliseconds.
Concealed within the bonnet hinges is an advanced system that can instantly lift the bonnet to create an energy-absorbing gap, reducing the force of an impact if a pedestrian is struck. It’s a feature you hope will never be used, but if it is, you want it working flawlessly. This is the Mercedes Active Bonnet system.
When an A-Class in Thatcham arrived at Fergies with this system out of action, it required expert investigation. Our award-winning team used extensive Mercedes experience, a clear step-by-step diagnostic method, and dealer-level tools to pinpoint and repair the problem first time. There were no random guesses, no unnecessary parts replacement, just a focused repair to return the system to full readiness.
Here’s how we traced the fault, confirmed the diagnosis beyond doubt, and sent this Mercedes back on the road with its pedestrian safety fully restored — without the dealership price tag.
How the Mercedes Active Bonnet Lifter System Helps Protect Pedestrians
The Active Bonnet system is a Mercedes safety feature designed to lessen the severity of injury if a pedestrian is struck. Built into the rear bonnet hinges are compact pyrotechnic devices, known as squibs, that can raise the back of the bonnet by around 80 mm in just milliseconds. This sudden lift creates a safety gap between the bonnet and the rigid engine components underneath, allowing the bonnet to absorb more of the impact force.
The system is managed by the car’s Supplemental Restraint System (SRS) control unit, which works together with dedicated impact sensors in the front bumper. When these sensors detect a collision pattern consistent with a pedestrian impact, they send the signal to fire the bonnet lifters. Each hinge has its own lifter, and once deployed, the bonnet remains in the raised position until it is reset. While you can push it back down to keep the vehicle on the road, the pedestrian protection function will not be restored until a professional resolves the fault.
Assessing the Active Bonnet Malfunction Warning (Fault Code: B2730)
When this Mercedes A-Class arrived at Fergies in Thatcham, the owner was concerned about a warning on the dashboard: “Active Bonnet Malfunction – see owner’s manual.” This isn’t the sort of alert to put off. If the system isn’t active, the vehicle loses a key pedestrian protection feature designed to react in milliseconds.
We connected our Mercedes dealer-level diagnostic tool and carried out a complete system scan. This is the same equipment trusted by Mercedes main dealers, giving us full access to official manufacturer data and guided test processes. The scan revealed a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In straightforward terms, the SRS control unit had detected an issue with the squib in the left-hand bonnet lifter.
While that narrowed down the area to investigate, a fault code is only the start of the process. To get to the true cause, we applied our structured diagnostic routine, comparing serial data, checking wiring continuity, inspecting connectors, and examining linked control modules, ensuring we pinpointed the precise fault and confirmed there were no other issues that could cause the warning to reappear.
Identifying the Cause of the Active Bonnet Malfunction
After the initial scan, we moved into a detailed investigation. At Fergies in Thatcham, this means applying a clear, step-by-step diagnostic process, using our extensive Mercedes experience alongside dealer-level data and specialist tools.
Our first step was to consult Mercedes-Benz’s official technical information for Fault Code B2730. This confirmed the fault related to the squib circuit in the left bonnet lifter. From there, our goal was to uncover why the system had logged the code, rather than simply assuming the lifter was the culprit.
We then compared serial data from the left and right bonnet lifter circuits. The right-hand circuit returned normal readings, while the left-hand side showed no signal; a clear sign of an open circuit. This kind of comparison is essential, as it allows the healthy side to act as a baseline, confirming that the fault is both genuine and isolated.
Next, we performed a thorough inspection of the wiring harness and connectors supplying the left bonnet lifter. We looked for corrosion, damage, or any loosened terminals, as even a small contact issue can cause faults in the SRS system. Continuity and resistance checks showed the wiring was in good condition and met all Mercedes-Benz specification limits.
With wiring and connectors ruled out, and serial data still showing no continuity for the left lifter squib, we could confirm the problem lay within the lifter itself. This evidence-led approach meant we replaced only the failed component, avoiding unnecessary cost and ensuring the pedestrian protection system was fully restored.
Carrying Out the Active Bonnet Malfunction Repair
With the source of the fault confirmed, we contacted the customer to explain our findings and the repair process in detail. This level of clarity is especially important when dealing with a safety-critical feature like the Active Bonnet. Once we had their approval, we followed Mercedes-Benz’s official safety steps for working with pyrotechnic components, which began with disconnecting the battery and allowing the SRS system to fully discharge before removing the faulty left-hand bonnet lifter.
The new lifter was installed following the manufacturer’s torque specifications to ensure a precise fit and secure mounting. We then compared both bonnet lifter circuits to verify that their readings matched. This is a crucial step. Without matched readings, the control unit can detect an imbalance and deactivate the system.
Finally, we carried out a thorough post-repair inspection. This included clearing the stored fault code, checking serial data to confirm the Active Bonnet system was operating exactly as it should, and ensuring there were no other associated warnings. Once every check was passed, the Mercedes was returned to its owner with the pedestrian protection system fully functional and ready if ever needed.
Mercedes A-Class Active Bonnet Malfunction Resolved
With the repair completed, this Mercedes A-Class left our Thatcham workshop free from its Active Bonnet malfunction warning and with the system operating exactly as Mercedes intended. As a crucial part of the car’s pedestrian protection technology, the active bonnet was once again prepared to deploy in milliseconds if required.
Our specialist Mercedes knowledge, combined with a clear, methodical diagnostic approach, meant we identified the fault precisely and replaced only the component that had failed. Backed by dealer-level technical information and specialist tools, we delivered a first-time fix that met the highest safety standards and offered excellent value for the customer.
For the driver, this wasn’t just about clearing a dashboard alert. It was about knowing that their Mercedes’ safety systems would respond instantly if needed, and having complete peace of mind that the repair had been carried out to the most exacting professional standards.
Ensure Your Mercedes Active Bonnet System Is Ready When Needed
If your Mercedes shows an Active Bonnet Malfunction warning, it’s important to address it quickly. This system is a key part of your vehicle’s pedestrian protection technology — and if it isn’t working, it won’t deploy in an emergency. If you notice a warning or suspect a problem, call the Mercedes experts at Fergies in Thatcham on 01635 778002.
Our award-winning team uses a proven diagnostic approach, supported by dealer-level tools, to locate faults with precision and ensure every repair meets Mercedes-Benz safety standards.
Other benefits of choosing Fergies include:
- Free courtesy car to keep you on the road
- 12-month parts and labour guarantee
- Dealer-level expertise without dealer prices
- Specialist experience with Mercedes safety systems
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Don’t take risks with your Mercedes’ safety. Call 01635 778002 today to arrange a professional inspection and have your vehicle’s safety systems restored to full readiness.
