Active Bonnet Malfunction
Mercedes doesn’t just put power under the bonnet — they’ve engineered rapid-response safety systems to protect in ways most drivers never see.
Hidden within the bonnet hinges is a mechanism that can lift the bonnet in milliseconds, creating space to absorb energy if a pedestrian steps into your path. It’s technology you hope you’ll never need, but if the moment comes, you want it working flawlessly. This is the Mercedes Active Bonnet system.
When this system failed on an A-Class in Cheltenham, it needed expert attention. At Cheltenham & Gloucester Autocentre, our award-winning team combined in-depth Mercedes knowledge with a methodical diagnostic process to find and fix the fault the first time. No trial-and-error, no unnecessary parts — just a precise repair that restored this safety-critical system.
Here’s how we identified the cause, confirmed it with absolute certainty, and returned this Mercedes to the road fully ready for those crucial split seconds… without the main dealer bill.
Understanding How the Mercedes Active Bonnet System Protects Pedestrians
The Active Bonnet system is a Mercedes safety innovation designed to help lessen the severity of injury in the event a pedestrian is struck. Positioned at the rear of each bonnet hinge are small pyrotechnic devices, known as squibs, which can raise the back edge of the bonnet by approximately 80 mm in a fraction of a second. This rapid lift creates a cushioning gap between the bonnet and the rigid engine parts beneath, allowing the bonnet to absorb impact energy more effectively.
The operation of this system is controlled by the car’s Supplemental Restraint System (SRS) module, which works alongside specialised sensors located in the front bumper. When these sensors detect a collision pattern matching a pedestrian impact, they signal the bonnet lifters to deploy. Each hinge has its own lifter, and once triggered, the bonnet remains elevated until reset. Although it can be lowered manually to allow continued driving, the pedestrian protection capability remains compromised until the underlying fault is professionally repaired.
Investigating the Active Bonnet Malfunction Warning (Fault Code: B2730)
When this Mercedes A-Class came into Cheltenham & Gloucester Autocentre, the owner was concerned by a dashboard warning: “Active Bonnet Malfunction – see owner’s manual.” This isn’t a message you can afford to ignore. If the system is disabled, that vital extra layer of pedestrian protection will not activate when needed most.
We connected our Mercedes dealer-level diagnostic tool and performed a full system scan. We use the same technology trusted by official Mercedes dealerships, giving us direct access to manufacturer data and guided test routines. The scan confirmed a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction.
Put simply, the SRS control unit had registered an issue with the squib for the left-hand bonnet lifter. The term “squib” refers to the small pyrotechnic device inside the actuator that fires to raise the bonnet.
While this pinpointed the area of concern, our experience tells us that a fault code is just the beginning. It highlights a symptom, not the root cause. To uncover the exact problem, we followed our structured, step-by-step diagnostic process — reviewing serial data, inspecting wiring, checking connectors, and verifying related control modules — to ensure we found the source of the issue and ruled out any other faults that could trigger the warning again.
Investigating the Source of the Active Bonnet Malfunction
After completing the initial scan, we moved into a full investigation. At Cheltenham & Gloucester Autocentre, that means following a clear, methodical diagnostic process, using our specialist Mercedes knowledge alongside dealer-level data and equipment.
We began by consulting Mercedes-Benz’s official technical documentation for Fault Code B2730. This confirmed that the code points to an issue in the squib circuit for the left bonnet lifter. From there, our objective was straightforward: determine exactly why the system had flagged the fault, rather than assuming the lifter itself was to blame.
Next, we compared serial data readings from the left and right bonnet lifter circuits. The right-hand side produced normal results, while the left-hand side showed no signal — a strong indicator of an open circuit. This side-by-side testing is invaluable, as it allows us to use the working side as a control, confirming that the issue is real and localised.
We then carried out a detailed inspection of the wiring harness and connectors serving the left bonnet lifter. This involved checking for signs of corrosion, physical damage, or loose terminals, as even minor contact issues can trigger a fault in the SRS network. Using continuity and resistance testing, we verified that the wiring could carry the correct signal and that all readings fell within Mercedes-Benz’s specified parameters. These tests confirmed that the wiring was sound and not contributing to the malfunction.
With the wiring and connectors in good condition, and serial data consistently showing no continuity for the left lifter squib, we could confirm the fault was inside the lifter itself. This measured, evidence-based approach ensured we replaced the correct part first time, avoiding unnecessary expense and restoring this important pedestrian protection system to full readiness.
How We Fixed the Active Bonnet Malfunction (Fault Code: B2730)
With the cause identified, we spoke to the customer to clearly explain our findings and the repair needed. Open communication is especially important when the work involves safety-critical systems like the Active Bonnet. Once we had their approval, we followed Mercedes-Benz’s strict safety guidelines for working on pyrotechnic components, which included disconnecting the battery and allowing the system to discharge fully before handling the faulty left-hand bonnet lifter.
The replacement lifter was fitted according to the manufacturer’s torque specifications to ensure correct alignment and secure fastening. We then compared both bonnet lifter circuits to make sure the readings matched. This step is essential — without it, the control unit could detect an imbalance and disable the system again.
Finally, we performed a comprehensive post-repair inspection. This involved clearing the stored fault code, verifying through serial data that the Active Bonnet system was operating correctly, and confirming there were no other related faults. Once every check had passed, the Mercedes was ready to return to the customer with its pedestrian protection system restored to full working order.
Active Bonnet Malfunction Resolved – Safety Restored
With the repair completed, this Mercedes A-Class left our Cheltenham workshop free from its Active Bonnet malfunction and with the system working exactly as intended. As a vital part of the car’s pedestrian protection technology, the active bonnet was once again ready to deploy in milliseconds should an incident occur.
Our specialist Mercedes knowledge, combined with a methodical testing process, ensured we pinpointed the fault accurately and replaced only the component that had failed. Supported by dealer-level technical data and the right tools, our customer received a first-time fix that met the highest safety standards while keeping costs under control.
For the driver, the benefits went far beyond clearing a dashboard light. It meant confidence that their Mercedes’ safety systems would respond instantly in an emergency, and peace of mind knowing the work had been carried out to the most exacting standards.
Active Bonnet Malfunction? Trust Our Mercedes Specialists
If your Mercedes shows an Active Bonnet Malfunction warning, it’s not something to delay. This feature plays a critical role in your vehicle’s pedestrian protection system — and if it’s inactive, the safety it’s designed to deliver won’t be there when you need it most. If you see a warning or suspect there’s a fault, call the Mercedes experts at Cheltenham & Gloucester Autocentre on 01452 855555.
Our award-winning team in Cheltenham uses a proven, methodical diagnostic process supported by dealer-level tools to identify the problem accurately first time, giving you complete confidence in the repair.
Other benefits of choosing Cheltenham & Gloucester Autocentre include:
- Free courtesy car to keep you mobile
- 12-month parts and labour guarantee
- Dealer-level expertise without dealer prices
- Specialist knowledge of Mercedes safety systems
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Don’t take chances with your Mercedes’ safety. Call us today on 01452 855555 to book a professional assessment and get your vehicle returned to full working order.
