Active Bonnet Malfunction
Mercedes doesn’t just build cars to perform — it builds them to protect. Hidden within the bonnet hinges is one of its cleverest safety innovations: the Active Bonnet system. In the split second of a collision, it lifts the bonnet clear to reduce the impact on a pedestrian. It’s the sort of feature you hope never gets used, but you’d want ready to fire in an instant if the unthinkable happens.
When an A-Class arrived at Shires Garage in Langport with an Active Bonnet Malfunction warning, it wasn’t a job for trial and error. Our team’s Mercedes knowledge, structured diagnostic process, and use of dealer-level equipment meant we could track down the fault quickly and fix it right the first time. No wasted parts, no guesswork — just a methodical repair that put safety back where it belonged.
Here’s how we uncovered the cause, verified the fault, and returned this Mercedes to the road with its pedestrian protection system restored — all without main dealer prices.
Mercedes Active Bonnet: The Hidden Safety Feature Explained
The Active Bonnet system is a clever safety feature fitted to the Mercedes A-Class, designed to reduce the risk of serious injury to pedestrians in a collision. Built into the bonnet hinges are small pyrotechnic lifters that can raise the rear edge of the bonnet by around 80 mm in a fraction of a second. By creating extra space between the bonnet and the hard engine components underneath, the system helps to absorb energy and lessen the severity of an impact on a pedestrian.
Control of the system comes from the car’s Supplemental Restraint System (SRS) unit, which works with crash sensors in the front bumper to detect a qualifying impact. Each hinge has its own lifter, and once triggered, the bonnet remains slightly raised until reset. While the bonnet can be pushed back down so the car can be driven, the protective function won’t return until the fault is repaired and the system is fully reset.
How We Began Investigating Fault Code B2730
When the Mercedes A-Class arrived at Shires Garage in Langport, the driver was worried about the dashboard alert: “Active Bonnet Malfunction – see owner’s manual.” This isn’t a warning you can afford to ignore. If the system isn’t functioning, that extra layer of pedestrian protection won’t be available should it be needed in an emergency.
We connected our Mercedes dealer-level equipment to the car’s onboard systems and carried out a full scan. This gives us the same level of information used by main dealers, ensuring we have full access to manufacturer data and guided test procedures. The results showed a stored trouble code: B2730 – Left Rear Engine Hood Lifter Squib Malfunction. In straightforward terms, the control unit had detected an issue with the small pyrotechnic lifter (the squib) on the left-hand side.
That gave us a starting point, but our experience has shown that a fault code is only the headline, not the whole story. A code identifies where the system has detected a problem, but not why it has occurred. To uncover the real cause, our technicians followed a structured, multi-step process — checking serial data, wiring condition, connectors, and linked control modules to confirm the exact failure point and ensure no hidden issues were waiting to reappear later.
Confirming What Caused the Active Bonnet Malfunction
With the first scan complete, we moved into a full investigation. At Shires Garage in Langport, that meant applying our structured, step-by-step diagnostic process, combining our Mercedes expertise with the same dealer-level data and tools that main dealerships rely on.
We began by checking Mercedes-Benz’s official guidance for Fault Code B2730, which confirmed the code relates to the squib circuit on the left bonnet lifter. Our job was clear — establish why the circuit had registered a fault rather than simply assuming the lifter itself was to blame.
Next, we compared serial data from the left and right bonnet lifter circuits. The right side showed normal readings, while the left side produced no signal at all, pointing towards an open circuit. This kind of side-by-side test is crucial; by using the healthy circuit as a reference, we could confirm the fault was genuine and not an intermittent issue.
We then carried out detailed checks on the wiring and connectors for the left-hand lifter. This included inspecting for corrosion or damage, followed by continuity and resistance tests to ensure the wiring could carry the correct signal. Everything tested within specification, meaning the wiring was sound.
With the wiring ruled out and the data consistently showing an open circuit at the left lifter squib, we concluded with confidence that the lifter itself had failed internally. This methodical process allowed us to identify the exact cause, replace only the faulty part, and avoid unnecessary costs — ensuring the Active Bonnet system could be fully restored.
How We Fixed the Fault in the Active Bonnet System
With the cause confirmed, we explained our findings to the customer and outlined the repair plan. When dealing with safety-critical systems like the Active Bonnet, transparency is essential. With approval given, we followed Mercedes-Benz’s strict safety procedures for handling pyrotechnic devices: disconnecting the battery and waiting for the system to fully discharge before removing the failed left-hand lifter.
The replacement lifter was installed carefully, torqued to the manufacturer’s specifications to guarantee correct alignment and secure fitting. Once fitted, we carried out a comparison between the left and right circuits. This step is vital; if the control unit still sees a discrepancy, it will continue to log a fault and disable the system.
We then performed a complete post-repair check. This included clearing the stored error code, confirming via serial data that the Active Bonnet was functioning correctly, and ensuring no related faults remained. With all tests passed, the A-Class was ready to leave Shires Garage with its pedestrian safety system fully operational once again.
Mercedes A-Class: Active Bonnet Fault Resolved
With the repair completed, this Mercedes A-Class left Shires Garage in Langport free of warning lights and with its Active Bonnet system restored to full working order. As part of the car’s pedestrian protection technology, the system was once again ready to react instantly should the worst happen.
Because our Mercedes specialists identified the fault precisely and replaced only the failed lifter, the repair was both accurate and cost-effective. Backed by dealer-level technical data and a structured process, we ensured the system was returned to manufacturer standards on the very first visit.
For the customer, it meant far more than simply clearing a dashboard message. It meant the reassurance of knowing their car’s safety systems would respond in milliseconds if ever needed, and the confidence that the work had been done right the first time.
Choose Shires Garage for Expert Mercedes Care
If your Mercedes displays an Active Bonnet malfunction warning light, it’s not something to delay. The Active Bonnet system is central to your vehicle’s pedestrian protection — and if it isn’t working, the safety it’s designed to provide won’t engage when it’s most needed. If you’ve noticed a warning on your dashboard, call the Mercedes experts at Shires Garage in Langport on 01458 250570.
Our team uses a structured diagnostic process supported by Mercedes dealer-level equipment to identify faults with accuracy the first time, giving you total confidence in your repair.
Other reasons drivers in Taunton and across Somerset choose Shires Garage include:
- Free courtesy car to keep you on the move
- 12-month parts and labour guarantee
- Dealer-level expertise without the dealer price tag
- Specialist tools and technical data for Mercedes safety systems
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Don’t take chances with your Mercedes’ safety. Call 01458 250570 today to arrange a professional inspection and restore your car’s protection system.
