Active Bonnet Malfunction
Mercedes have a way of engineering safety into places you wouldn’t expect — even the bonnet hinges.
Hidden there is a system that can respond faster than the blink of an eye, lifting the bonnet instantly in a collision to soften the blow if a pedestrian steps out ahead. It’s a safety feature you hope never to need… but you’d want it fully functional if you did. This is the Active Bonnet system.
When that system failed on a Mercedes A-Class in Yeovil, it was a job for genuine expertise. At Robert Cockings Motor Repairs, our award-winning team used proven Mercedes know-how, advanced diagnostic methods, and dealer-level equipment to track down the fault and carry out the correct repair the first time. No wasted parts, no trial and error — just precision work that restored the car’s pedestrian protection.
Here’s how we diagnosed the issue, confirmed it with certainty, and returned the Mercedes to its owner with safety restored… and all without the main dealer price.
The Mercedes Active Bonnet System Explained
Mercedes developed the Active Bonnet system to help reduce pedestrian injuries if the worst should happen. Built into the rear bonnet hinges are small pyrotechnic devices called squibs, which fire in an instant to lift the bonnet by around 80mm. This sudden movement creates a vital gap between the bonnet and the engine’s solid components, allowing it to absorb energy and lessen the severity of impact should a pedestrian be hit.
The system is managed by the car’s Supplemental Restraint System (SRS) control unit, which works with crash sensors mounted in the front bumper to detect a qualifying collision. Each side of the bonnet has its own lifter, and once deployed, the bonnet remains slightly raised until reset. While it can be pushed down so the car can still be driven, the protection is no longer in place until the fault is corrected.
Active Bonnet Malfunction: Our First Diagnostic Steps
When the Mercedes A-Class arrived at Robert Cockings Motor Repairs, the owner was concerned about the dashboard message: “Active Bonnet Malfunction – see owner’s manual.” This is never something to overlook. Without the system operating, the extra layer of pedestrian protection isn’t there if it’s suddenly needed.
We connected our Mercedes dealer-level diagnostic equipment to the car and ran a full scan of the onboard systems. This is identical to the setup used by main dealerships, giving us access to manufacturer data and guided test plans. The scan flagged a stored code: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In straightforward terms, the SRS control unit had recorded an issue with the squib in the left bonnet lifter.
Although this identified the area of the fault, our Mercedes-trained technicians know that a code is only part of the story. It shows where the system has recognised a problem, but not what caused it. To be sure, we carried out our structured diagnostic routine: reviewing serial data, testing wiring integrity, inspecting connectors, and checking other linked control modules to confirm the exact source of the fault and to rule out any hidden problems that might resurface later.
How We Verified the Active Bonnet System Fault
With the scan results noted, our next step was to carry out a thorough investigation. At Robert Cockings Motor Repairs in Yeovil, that meant applying our structured diagnostic routine, supported by Mercedes expertise and the same dealer-level equipment and technical information relied on by the manufacturer.
We began by consulting the official Mercedes 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 goal was clear: establish why the circuit was faulty, not simply assume the lifter needed replacing.
To narrow it down, we compared serial data from the left and right lifter circuits. The right-hand circuit operated normally, while the left showed no signal at all — a classic sign of an open circuit. Comparing both sides was vital, as the healthy circuit gave us a benchmark and confirmed the problem was real and localised.
We then turned to the wiring harness and connectors feeding the left lifter. After checking for loose terminals, corrosion, or visible damage, we carried out continuity and resistance tests to confirm the wiring could handle the correct signal. All values checked out, showing the wiring was healthy.
With the wiring ruled out and the serial data still showing an open circuit, the conclusion was clear: the left bonnet lifter had failed internally. This step-by-step approach ensured we targeted the right component first time, avoided unnecessary parts replacement, and confirmed the safety-critical system could be put back into full working order.
Active Bonnet Repair: Fault Code B2730 Resolved
With the diagnosis complete, we contacted the customer to explain the results and how the repair would be carried out. Being transparent is especially important when working on safety-critical technology like the Active Bonnet system. Once we had their approval, we followed Mercedes’ safety procedures for handling pyrotechnic devices: disconnecting the battery and allowing the system to discharge fully before removing the faulty left-hand bonnet lifter.
The replacement lifter was then installed to the correct Mercedes torque settings, ensuring correct alignment and secure fitting. We then carried out a comparison between the left and right bonnet lifter circuits. This step cannot be skipped — if not performed, the control unit may still register a fault and deactivate the system.
Finally, we performed a full post-repair check. This included clearing the fault memory, confirming via serial data that the Active Bonnet system was once again operational, and checking that no related faults remained. With all tests completed successfully, the Mercedes was ready to return to its owner with the pedestrian protection feature fully restored.
Active Bonnet Malfunction Corrected and Verified Safe
Once the repair was finalised, this Mercedes A-Class left Robert Cockings Motor Repairs in Yeovil with the Active Bonnet warning cleared and the system back to operating exactly as designed. As a vital part of the car’s pedestrian protection package, the Active Bonnet was once again ready to deploy instantly if the situation ever demanded it.
Because our technicians identified the precise fault and replaced only the defective lifter, the repair was carried out with accuracy and efficiency. Guided by official Mercedes data, dealer-level tools, and a structured testing process, we ensured the system was restored to the manufacturer’s safety standards the first time.
For the driver, this meant more than the disappearance of a warning message. It gave them confidence that their Mercedes’ pedestrian protection system would react in milliseconds if required, and reassurance that the repair had been performed accurately and to the highest standard.
Mercedes Safety Repairs You Can Trust in Yeovil
If your Mercedes shows an Active Bonnet Malfunction warning, it’s not something to ignore. This system is central to your car’s pedestrian protection technology, and if it isn’t working, the safety response it’s built to provide simply won’t activate in a collision. If you see a warning, contact the Mercedes specialists at Robert Cockings Motor Repairs on 01935 427139.
Our Mercedes experts in Yeovil use a structured diagnostic approach, backed by dealer-level tools and manufacturer data, to identify faults correctly the first time and ensure your vehicle is repaired with precision.
Choosing Robert Cockings Motor Repairs also gives you:
- Free courtesy car to keep you moving
- 12-month parts and labour guarantee
- Award-winning service (Top Garage 2022)
- Dealer-level knowledge without the dealer price tag
- Specialist access to Mercedes safety information and equipment
Our commitment to quality is reflected in our {{average-rating}} star Google rating from {{review-count}} satisfied motorists.
Don’t take chances with your Mercedes safety systems. Call Robert Cockings Motor Repairs on 01935 427139 today to book your technical assessment and get your car back to full working order.
