Active Bonnet Malfunction
Mercedes vehicles are built to deliver more than driving performance — they’re designed with innovative safety systems working silently in the background. One of these is the Active Bonnet, a feature hidden within the bonnet hinges that can raise the panel in milliseconds to create a safety gap if a pedestrian is struck. It’s the kind of technology you hope never needs to activate, but one you’d want in perfect working order if it ever does.
When this system developed a fault on a Mercedes A-Class brought to Protech Automotive in Newport, it required a methodical, expert approach. Using our specialist knowledge of Mercedes safety technology, structured diagnostic procedures, and manufacturer-grade equipment, we located the precise cause of the fault and resolved it without replacing parts unnecessarily. This ensured the repair was accurate, cost-efficient, and returned the vehicle’s pedestrian protection to full readiness.
Here’s how we traced the problem, confirmed our findings with certainty, and returned this Mercedes to the road with its safety system fully operational.
Explaining the Mercedes Active Bonnet System
The Mercedes Active Bonnet system is designed to reduce the severity of injuries to pedestrians in an impact. Built into the bonnet’s rear hinges are small pyrotechnic devices, known as squibs, which can raise the trailing edge of the bonnet by about 80 mm in a fraction of a second. This creates an energy-absorbing gap between the bonnet panel and the hard engine parts beneath, allowing the bonnet to deform more gradually on impact.
As part of the vehicle’s Supplemental Restraint System (SRS), the Active Bonnet works in conjunction with impact sensors located in the front bumper. When these sensors detect the specific deceleration pattern of a pedestrian collision, they send a signal to the SRS control unit, which fires the squibs and activates the lifters.
Each hinge contains its own lifter, so either side can deploy independently. Once activated, the bonnet stays in a raised position until the relevant components are replaced or reset. While you can push the bonnet down to continue driving, the system’s safety function remains inactive until a full repair is carried out — making it important to address any warning promptly.
The First Stage of Fault Code B2730 Diagnosis
When a Mercedes A-Class was brought to Protech Automotive in Newport with an “Active Bonnet Malfunction – see owner’s manual” warning displayed, we knew its pedestrian protection system wouldn’t operate if it were called upon. As this feature is designed to activate in milliseconds during a qualifying impact, any fault must be addressed without delay.
Our first stage was to connect the vehicle to our Mercedes-specific diagnostic equipment and perform a full system scan. This dealer-level technology provides access to manufacturer data and guided test routines, ensuring our findings are accurate and model-specific. The scan recorded fault code B2730, signalling an electrical issue in the left rear bonnet lifter squib circuit.
However, identifying the location of the issue is only part of the process — it doesn’t explain why the system has failed. To find the underlying cause, we followed our methodical diagnostic process, comparing serial data from each bonnet lifter, checking connectors and wiring for damage or corrosion, and confirming the performance of related control modules. This ensured we had a complete and reliable picture before proceeding to the repair.
Tracking Down the Fault in the Mercedes Active Bonnet
With fault code B2730 confirmed, Protech Automotive began a detailed investigation to determine exactly why the Active Bonnet system had stopped functioning. Using our methodical diagnostic approach and Mercedes-specific technical data, we worked step-by-step to ensure nothing was overlooked.
Mercedes guidance states that B2730 refers to an electrical fault in the left rear bonnet lifter squib circuit. While this told us where the fault had been detected, our priority was to uncover the reason behind it.
We compared serial data from both bonnet lifter circuits. The right-hand side readings were within the expected range, but the left-hand side returned no measurable signal, strongly indicating an open circuit. This confirmed the fault was confined to the left-hand side.
We then carried out a close inspection of the wiring harness and connectors leading to the left lifter. Checks for corrosion, damaged pins, or loose connections came back clear, and continuity and resistance tests showed the wiring was operating to manufacturer specifications.
With the wiring and connectors ruled out, and serial data continuing to indicate an open circuit, we concluded that the left bonnet lifter itself was the source of the fault. This allowed us to proceed with a targeted repair, avoiding any unnecessary part replacements.
How We Carried Out the Active Bonnet Repair
With the diagnosis confirmed, we explained our findings to the customer at Protech Automotive in Newport. Clear communication is key when repairing critical safety systems, so we detailed the repair process before starting any work.
Following Mercedes-Benz’s official safety guidelines for handling pyrotechnic components, we disconnected the battery and waited for the system to discharge fully. The defective left-hand bonnet lifter was then removed, starting with the wiring harness disconnection before unbolting it from its mountings.
We installed a new lifter, aligning it precisely and tightening it to the correct torque values specified by Mercedes. Once fitted, we compared serial data from both bonnet lifters to confirm the repaired side was delivering the same readings as the fully functional side — a crucial step to avoid further fault logging.
After clearing the B2730 code from the SRS control unit, we carried out a complete system test. The “Active Bonnet Malfunction” warning was gone, and the pedestrian protection system was confirmed as fully operational.
Pedestrian Protection Restored with Active Bonnet System Repair
Following the repair at Protech Automotive in Newport, this Mercedes A-Class had its Active Bonnet system fully operational once again. The B2730 fault code was cleared, the dashboard warning removed, and serial data confirmed that both bonnet lifters were performing in perfect sync.
With the pedestrian protection system reinstated, the car is once again capable of deploying the Active Bonnet in milliseconds if needed — restoring an important safety feature designed to minimise injury in collisions.
By identifying the fault with precision and replacing only the defective component, we delivered a cost-effective solution that met Mercedes’ safety standards. For the owner, it meant more than fixing a fault; it brought peace of mind that their vehicle’s safety technology is ready to act.
Protech Automotive – Restoring Your Mercedes Pedestrian Protection
If your Mercedes shows an “Active Bonnet Malfunction” warning, its pedestrian protection feature isn’t ready to operate. This safety system is designed to activate in milliseconds during specific collisions, so prompt repair is essential.
At Protech Automotive in Newport, we use Mercedes-specific diagnostic tools and a methodical testing process to pinpoint faults with precision. All repairs are completed to manufacturer standards and backed by our 12-month parts and labour guarantee, ensuring your vehicle’s safety system is restored to full readiness.
Why choose Protech Automotive?
- 12-month parts and labour guarantee on all repairs
- Courtesy car available to keep you mobile
- In-depth knowledge of Mercedes safety systems and fault codes
- Dealer-level diagnostic capabilities without dealer prices
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Call Protech Automotive on 01633 213713 to have your Mercedes Active Bonnet system repaired by trusted local specialists.
