Active Bonnet Malfunction
Mercedes design safety features where you’d least expect them — even in the bonnet hinges.
Concealed there is technology that reacts faster than a blink, raising the bonnet in an instant if a collision occurs, softening the impact should a pedestrian be struck. It’s the kind of system you hope will never be called upon… but you’d want operating perfectly if it ever was. This is the Active Bonnet system.
When that system showed a fault on a Mercedes A-Class in Wellingborough, it wasn’t a case for trial and error. At SB Autocare, our technicians applied Mercedes knowledge, step-by-step diagnostic checks, and dealer-level equipment to confirm the problem and resolve it first time. No wasted parts, no guesswork — just a precise repair that restored the vehicle’s safety system.
Here’s how we uncovered the issue, confirmed it beyond doubt, and returned the Mercedes A-Class ready to protect in the moments that count… and all without the main dealer expense.
Mercedes Active Bonnet: Designed for Protection
The Active Bonnet system is a safety measure engineered by Mercedes to help reduce pedestrian injuries during a collision. At the rear bonnet hinges are small pyrotechnic devices called squibs, which fire in a split second to raise the bonnet by around 80mm. This sudden lift creates valuable space between the bonnet and the engine’s solid components, allowing it to absorb energy and lessen the impact if a pedestrian is struck.
Oversight of the system comes from the car’s Supplemental Restraint System (SRS) control unit, working with crash sensors fitted in the front bumper to recognise a qualifying impact. Each hinge is equipped with its own lifter, and once deployed, the bonnet remains partially raised until reset. Although the bonnet can be pushed back down to keep the car drivable, the protective function is lost until the repair is completed.
Active Bonnet Malfunction: First Stage of Our Investigation
Recently, a Mercedes A-Class was booked into SB Autocare with a dashboard alert: “Active Bonnet Malfunction – see owner’s manual.” This is not the kind of message to ignore, and the owner was rightly concerned. If the system isn’t operational, the additional pedestrian protection it provides won’t be available in an emergency.
We connected our Mercedes dealer-level diagnostic equipment to the car and carried out a full system scan. This is the same type of tool used in main dealerships, which gives us access to official manufacturer data and guided test routines. The scan revealed a stored trouble code: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In simple terms, the SRS module had identified a fault in the squib fitted to the left bonnet lifter.
But a fault code is only a starting point. Our Mercedes-trained specialists know it tells you where the problem has been detected, not why it has occurred. To get to the root cause, we worked through our methodical diagnostic process: analysing serial data, inspecting the wiring, checking connectors, and testing associated control modules to make sure there were no additional issues waiting to appear later.
Active Bonnet Malfunction: The Real Issue Identified
With the initial scan complete, it was time to dig deeper. At SB Autocare in Wellingborough, that meant applying our structured diagnostic routine, using Mercedes know-how along with the same dealer-level tools and technical data relied on by the manufacturer.
Our starting point was to consult Mercedes-Benz’s official documentation for Fault Code B2730. This confirmed the code pointed to the squib circuit in the left bonnet lifter. From there, our task was clear: establish why the circuit had failed, rather than jump to the conclusion that the lifter itself was the problem.
We then carried out a comparison of serial data between the two bonnet lifter circuits. The right-hand circuit behaved normally, while the left produced no signal at all — the hallmark of an open circuit. This side-by-side test was vital, as the healthy circuit gave us a reference point and confirmed the fault was both genuine and isolated.
Next, we examined the wiring harness and connectors for the left-hand lifter. We looked for signs of corrosion, loose pins, or physical damage, then performed continuity and resistance checks to confirm the wiring was capable of carrying the correct signal. All the readings came back within specification, proving the wiring was sound.
With wiring and connectors eliminated as causes, and serial data consistently showing an open circuit at the left-hand lifter, the conclusion was clear: the lifter itself had failed internally. By following this structured process, we pinpointed the fault accurately the first time, avoided guesswork or unnecessary part replacements, and ensured this safety-critical system could be fully restored.
Repairing the Active Bonnet System and Restoring Pedestrian Safety
Once the diagnosis was confirmed, we contacted the customer to explain our findings and outline the repair process. Being clear and open is especially important when working on systems as safety-critical as the Active Bonnet. With their approval, we followed Mercedes’ official safety procedures for pyrotechnic devices: disconnecting the battery, waiting for the system to discharge, and then carefully removing the faulty left-hand bonnet lifter.
The new lifter was fitted to the manufacturer’s specified torque settings, ensuring secure installation and accurate alignment. We then checked both bonnet lifter circuits against each other. This step is essential — without it, the control unit could continue to log a fault and keep the system disabled.
Finally, we carried out a full post-repair validation. This included clearing the stored code, confirming via serial data that the Active Bonnet system was reporting fully operational, and double-checking that no related issues were present. With every safety check completed successfully, the Mercedes A-Class was returned to its driver with pedestrian protection fully restored.
Error Code B2730 Cleared – Active Bonnet System Restored
With the repair completed, this Mercedes A-Class left SB Autocare in Wellingborough with Error Code B2730 cleared and the Active Bonnet system back to full working order. As an integral part of the vehicle’s pedestrian protection package, the bonnet lifter system was once again ready to react instantly in the event of a collision.
By diagnosing the exact cause and replacing only the failed lifter, our technicians delivered a precise, cost-efficient repair. Supported by Mercedes technical data, a structured test process, and dealer-level tools, we ensured the system was fully reinstated on the first visit, meeting strict safety expectations.
For the owner, the benefit was more than just clearing a dashboard warning. It meant confidence that their Mercedes’ pedestrian safety system would deploy in milliseconds if needed, along with the peace of mind that the repair had been carried out to manufacturer standards.
SB Autocare Wellingborough: Mercedes Safety Experts
If your Mercedes displays an Active Bonnet Malfunction warning, it’s not something to delay. This system is a vital part of your vehicle’s pedestrian safety technology, and if it isn’t working, the protection it’s designed to deliver simply won’t activate in an emergency. If you see this warning, contact the Mercedes specialists at SB Autocare on 01933 229630.
Our expert team in Wellingborough follow a structured diagnostic process, backed by dealer-level tools and Mercedes technical data, to identify the root cause correctly the first time and give you complete confidence in the repair.
Choosing SB Autocare also provides:
- Free courtesy car to keep you on the road
- 12-month parts and labour guarantee
- Dealer-level expertise without the dealership prices
- Access to Mercedes-specific safety equipment and knowledge
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Don’t risk your Mercedes safety systems. Call SB Autocare on 01933 229630 today to book a professional evaluation and restore your Active Bonnet system to full working order.
