Active Bonnet Malfunction
Mercedes engineers pack far more than just power beneath the bonnet — they’ve built in protection systems designed to react in fractions of a second.
Tucked neatly into the hinges is advanced technology that can instantly lift the bonnet, creating a protective buffer if the worst were to happen and a pedestrian stepped unexpectedly into the car’s path. It’s an innovation you hope will never activate, but if it does, you want absolute confidence that it will function exactly as intended. This is the Mercedes Active Bonnet system.
When this system failed on an A-Class in Ulverston, it called for more than guesswork. The skilled team at Athersmith Motor Services drew on in-depth Mercedes knowledge, a systematic fault-finding approach, and years of hands-on experience to identify the fault with accuracy and put it right on the first attempt. No random part replacements, no trial-and-error — just precise diagnosis and a targeted repair to return the car to full pedestrian safety standards.
Here’s how we located the cause, confirmed the problem beyond doubt, and restored this Mercedes to full readiness for those critical moments… without the dealership prices.
How the Mercedes Active Bonnet System Works to Protect Pedestrians
The Active Bonnet system is a Mercedes safety innovation designed to reduce the severity of injury if a pedestrian is struck. At the rear of each bonnet hinge are compact pyrotechnic devices, known as squibs, that instantly raise the back edge of the bonnet by roughly 80 mm. This rapid movement creates an energy-absorbing gap between the bonnet panel and the hard engine parts below, allowing the space to absorb the impact.
Control of the system lies with the vehicle’s Supplemental Restraint System (SRS) unit, which is linked to specialised impact sensors positioned within the front bumper. These sensors detect collision patterns consistent with a pedestrian impact and send a signal to deploy the bonnet lifters. Each hinge is equipped with its own lifter, and once activated, the bonnet remains in its raised position until it is reset. While you can lower it manually to continue driving, the pedestrian protection function is compromised until the underlying fault is fully addressed.
Our First Assessment of the Active Bonnet Warning (Fault Code: B2730)
When this Mercedes A-Class arrived at Athersmith Motor Services, the owner was worried about a dashboard message reading: “Active Bonnet Malfunction – see owner’s manual.” This isn’t the kind of alert you can overlook. If the system is inactive, that extra layer of pedestrian safety won’t deploy when it’s most needed.
We connected our Mercedes dealer-level diagnostic tool to the car’s systems and carried out a full scan. We use the same equipment that official Mercedes dealerships rely on, giving us direct access to manufacturer-specific data and guided test routines. The scan revealed a stored fault: B2730 – Left Rear Engine Hood Lifter Squib Malfunction.
While this told us where the vehicle had detected a fault, experience teaches our technicians that a fault code is just the starting point. It flags a symptom, not the cause. To uncover the real problem, we applied a structured, step-by-step diagnosis, reviewing serial data, verifying wiring integrity, inspecting connectors, and checking linked control modules. This ensured we pinpointed the precise failure and ruled out any other hidden issues that could trigger the fault again.
Tracing the Cause of the Active Bonnet Malfunction
Once the initial scan was complete, we moved into a thorough investigation. At Athersmith Motor Services in Ulverston, that means following a proven, step-by-step diagnostic routine, combining our Mercedes expertise with dealer-level technical data and specialist equipment.
We began by reviewing Mercedes-Benz’s official technical resources for guidance on Fault Code B2730. These confirmed that the code refers to a problem within the squib circuit for the left bonnet lifter. The goal from here was clear: determine why the system had logged the fault, rather than simply assuming the lifter itself was defective.
Next, we compared live serial data readings from both the left and right bonnet lifter circuits. The right-hand side operated as expected, but the left-hand side produced no signal at all — a clear indicator of an open circuit. Running this kind of side-by-side check is invaluable; by using the working side as a reference, we could be certain the issue was genuine and isolated.
We then carried out a close inspection of the wiring loom and connectors supplying the left bonnet lifter, checking for signs of corrosion, physical damage, or loose terminals. Continuity and resistance testing confirmed the wiring was functioning within Mercedes’ specified limits.
With the wiring and connectors verified as healthy, and serial data consistently showing no continuity in the left lifter squib, we concluded with confidence that the fault lay within the lifter itself. This careful, methodical process ensured we replaced the correct part first time, avoiding unnecessary expense while restoring this critical pedestrian protection feature to full readiness.
Repairing the Mercedes-Benz Active Bonnet Malfunction
With the cause identified, we contacted the customer to clearly explain our findings and the repair required. Clear communication is essential, particularly when the work involves safety-critical systems like the Active Bonnet. With the customer’s approval, we followed Mercedes-Benz’s strict safety procedures for working with pyrotechnic components, which included disconnecting the battery and allowing the system to fully discharge before touching the faulty left-hand bonnet lifter.
The replacement lifter was fitted in line with the manufacturer’s torque specifications to ensure secure mounting and correct alignment. We then rechecked both bonnet lifter circuits to confirm the system was reading correctly. This step is vital, as any imbalance between the two sides could cause the control unit to flag another fault and disable the system.
Finally, we completed a comprehensive post-repair inspection. This involved clearing the stored fault code, verifying through serial data that the Active Bonnet system was operating as intended, and confirming there were no other related warnings. Once every check was passed, the Mercedes was ready to return to its owner with full pedestrian protection reinstated.
Mercedes A-Class Restored to Full Safety Standards
With the repair completed, this Mercedes A-Class left our Ulverston workshop free from its Active Bonnet Malfunction warning and with the system fully operational once again. As a key part of the vehicle’s pedestrian protection technology, the active bonnet was now ready to respond in milliseconds if an incident occurred.
Our specialist Mercedes knowledge, combined with dealer-level technical data, precision equipment, and a structured testing process, meant we could deliver an accurate diagnosis and replace only the component that had failed. This approach provided a cost-effective, first-time fix while meeting the highest safety requirements.
For the driver, this wasn’t just about clearing a dashboard message. It was about the reassurance that their Mercedes’ safety systems would perform exactly as intended should the worst happen, and the confidence that the work had been carried out to manufacturer standards.
Keep Your Mercedes Safe On The Road With Athersmith Motor Services
If your Mercedes displays an Active Bonnet Malfunction warning, it’s not something to delay. This feature is central to your vehicle’s pedestrian safety systems — and if it’s inactive, the protection it’s designed to deliver simply won’t be there in an emergency. If you notice a warning or suspect something isn’t right, call our Mercedes specialists at Athersmith Motor Services on 01229 585288.
Our team in Ulverston follows a proven, methodical diagnostic process, backed by dealer-level tools, to identify faults with precision the first time, giving you complete confidence in the repair.
Other reasons to choose Athersmith Motor Services include:
- 12-month parts and labour guarantee
- Dealer-level expertise without dealer prices
- Specialist knowledge in Mercedes safety systems
Our Google rating of {{average-rating}} stars is based on feedback from {{review-count}} happy customers.
Don’t compromise on your Mercedes’ safety. Call us today on 01229 585288 to arrange a professional evaluation and get your vehicle returned to full working order.
