Active Bonnet Malfunction
Mercedes builds more than performance under the bonnet — they design intelligent safety systems that react faster than the blink of an eye.
Integrated into the bonnet hinges is advanced technology that can instantly raise the bonnet to create a protective gap, softening the impact if a pedestrian crosses your path. It’s a feature you never want to use, but if the need arises, you want it ready to work perfectly. This is the Mercedes Active Bonnet system.
When an A-Class in Bedford arrived at Cole & Miller Automotive with this system inactive, it called for accurate fault-finding. Our skilled technicians drew on years of experience, specialist Mercedes insight, and a systematic diagnostic approach to resolve the fault on the first attempt. No guesswork, no needless replacements, only a targeted repair to restore pedestrian safety to full function.
Here’s how we tracked the fault to its source, confirmed the issue without doubt, and returned this Mercedes ready to protect when it matters most… all without dealership pricing.
The Role of the Mercedes Active Bonnet System in Pedestrian Safety
The Active Bonnet system is a Mercedes safety feature designed to reduce the risk of serious injury if a pedestrian is struck. Built into the rear hinges of the bonnet are compact pyrotechnic units, known as squibs, which can instantly raise the back of the bonnet by about 80 mm. This sudden lift creates a protective gap between the bonnet and the solid engine components below, allowing the bonnet extra crumple space to absorb more of the impact energy.
This system is managed by the vehicle’s Supplemental Restraint System (SRS) control unit, which operates in tandem with dedicated impact sensors in the front bumper. If these sensors detect collision forces consistent with hitting a pedestrian, 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 it can be pushed back down to keep the car driveable, the pedestrian protection function stays inactive until the fault is professionally resolved.
Initial Diagnosis of the Active Bonnet Malfunction Alert (Fault Code: B2730)
When this Mercedes A-Class arrived at Cole & Miller Automotive, the owner reported a dashboard alert: “Active Bonnet Malfunction – see owner’s manual.” This is not a warning you can brush aside. If the system isn’t active, your Mercedes loses an important pedestrian safety feature designed to respond in milliseconds.
We connected our Mercedes dealer-level diagnostic tool and ran a complete scan of the vehicle’s systems. This is the same equipment used in main Mercedes dealerships, giving us full access to factory data and guided test routines. The scan returned a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction.
Although the code showed us where the problem had been identified, our experience proves that a fault code is only the starting point. It indicates a symptom, not the underlying cause. To determine the real fault, we used our methodical multi-step diagnostic approach, analysing serial data, testing wiring integrity, inspecting connectors, and checking linked control modules, ensuring we uncovered the precise source and confirmed there were no other hidden issues.
Finding What’s Behind the Active Bonnet Malfunction
With the initial scan complete, we moved on to a thorough investigation. At Cole & Miller Automotive, this means applying a structured, step-by-step diagnostic process, combining our Mercedes expertise with access to dealer-level technical data and tooling.
Our first step was to review Mercedes-Benz’s official information for Fault Code B2730, which confirmed it relates to a fault in the squib circuit for the left bonnet lifter. Knowing this, our goal was clear: establish the reason the system had logged the fault, rather than jumping to conclusions about the lifter being defective.
We then compared serial data readings from both bonnet lifter circuits. The right-hand circuit behaved normally, while the left-hand circuit produced no signal at all; a clear indication of an open circuit. This type of comparison is essential, as the functioning side can be used as a reference point to verify that the fault is genuine and isolated.
Next, we performed a detailed check of the wiring harness and connectors leading to the left bonnet lifter. We looked for corrosion, damage, or loose terminals, as even a slight connection fault can cause issues in the SRS system. Continuity and resistance testing confirmed the wiring was capable of transmitting the correct signal, and all readings matched Mercedes-Benz’s specified tolerances.
With the wiring and connectors confirmed as healthy, and serial data still showing no continuity on the left lifter squib, we were confident the lifter itself had failed internally. This methodical approach ensured we replaced the right part first time, avoiding unnecessary expense and restoring this vital pedestrian protection feature to full working order.
Active Bonnet Malfunction Repair: Getting the System Back to Full Function
With the fault confirmed, we contacted the customer to explain exactly what we had found and how we planned to resolve it. Clear explanation matters, particularly when the repair involves essential safety equipment like the Active Bonnet. Once we had the go-ahead, we worked in line with Mercedes-Benz’s safety procedures for handling pyrotechnic parts, disconnecting the battery and allowing the system to discharge completely before removing the faulty left-hand bonnet lifter.
The new lifter was installed to the manufacturer’s torque specifications, ensuring both correct fitment and secure mounting. We then tested both bonnet lifter circuits to confirm they were delivering matching readings. This check is vital, as any imbalance could cause the control unit to register another fault and deactivate the system.
We finished with a full post-repair evaluation, which included clearing the stored fault code, confirming that the Active Bonnet system was functioning correctly, and ensuring no other related issues were present. With every safety test passed, the Mercedes was returned to its owner, ready to provide full pedestrian protection if ever required.
Full Pedestrian Protection Restored After Active Bonnet Repair
Once the repair was complete, this Mercedes A-Class left our Arlesey workshop with no Active Bonnet Malfunction warning light and the system fully operational once again. As a critical element of the vehicle’s pedestrian protection setup, the active bonnet was ready to activate in a fraction of a second if the worst happened.
Drawing on our extensive Mercedes expertise and structured diagnostic approach, we located the exact cause of the fault and replaced only the part that had failed. With the backing of dealer-level technical resources and specialist equipment, we achieved a first-time fix that met the most rigorous safety requirements and offered excellent value for the customer.
For the customer, the result was more than the removal of an alert on the dashboard. It was the reassurance that their Mercedes’ safety systems were ready to perform exactly as intended in a critical moment, and the confidence that the repair had been completed to exacting professional standards.
Your Local Experts for Active Bonnet Malfunction Repairs
If your Mercedes is showing an Active Bonnet Malfunction warning, it’s not something you can afford to ignore. This system is an essential part of your car’s pedestrian safety features, and if it’s not working, it won’t protect you or others in an emergency. If you notice a warning light or think there may be an issue, call the Mercedes specialists at Cole & Miller Automotive on 01462 730803.
Our skilled team in Arlesey follows a clear, evidence-led diagnostic process, supported by dealer-level equipment and information, to pinpoint faults accurately first time. That means you can trust every repair to meet the highest safety standards.
Other reasons to choose Cole & Miller Automotive include:
- Free courtesy car while we work on your vehicle
- 12-month parts and labour guarantee
- Dealer-level expertise without the dealership prices
- Specialist knowledge of Mercedes safety systems
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Don’t compromise on your Mercedes’ safety. Call us today on 01462 730803 to arrange a professional inspection and get your vehicle fully restored.
