Active Bonnet Malfunction
Mercedes vehicles are built with more than performance in mind, integrating safety technology that works in the background to protect people in critical moments.
Hidden within the bonnet hinges is the Active Bonnet system, capable of lifting the bonnet in a fraction of a second to create extra space if a pedestrian is hit. It is a feature you may never see in action, but one you would want ready to function perfectly if the need arose.
When the Active Bonnet system on a Mercedes A-Class in Bridgwater developed a fault, there was no place for guesswork. At Somerset Vehicle Services, our hands-on Mercedes experience, methodical fault-finding, and use of manufacturer-grade diagnostic tools meant we could pinpoint the root cause and restore the system without replacing parts unnecessarily. This approach kept repair costs under control while ensuring the pedestrian protection system was fully operational.
Here is how we identified the fault with certainty and returned this Mercedes to the road with its safety technology back to full readiness, all without the main dealer’s price tag.
What Makes the Active Bonnet System So Important?
The Active Bonnet system in a Mercedes exists for one critical reason: to help minimise injury to pedestrians in the event of a collision. Discreetly installed in the rear bonnet hinges are small pyrotechnic units known as squibs. When activated, they raise the back edge of the bonnet by about 80 mm in just milliseconds. This rapid lift creates a cushioning gap between the bonnet and the rigid engine components beneath, allowing the panel to deform more gradually and absorb more of the impact energy.
This advanced safety feature forms part of the vehicle’s Supplemental Restraint System (SRS) and works with dedicated sensors located in the front bumper. These sensors are programmed to detect the specific forces associated with a pedestrian strike, sending an immediate signal to the SRS control unit to fire the squibs and lift the bonnet.
Both hinges are equipped with their own lifter, and once deployed, the bonnet remains slightly raised until the components are replaced or reset. While you can push the bonnet back into place to keep driving, the pedestrian protection capability is disabled until the system is fully repaired. That’s why any “Active Bonnet Malfunction” message should be addressed quickly — it’s not just another warning light, but a sign that one of the car’s key safety mechanisms isn’t ready to do its job.
The Initial Investigation into the Active Bonnet Issue
When a Mercedes A-Class arrived at Somerset Vehicle Services, the owner pointed out a persistent dashboard message: “Active Bonnet Malfunction – see owner’s manual.” This isn’t something that can be ignored, as it means the car’s pedestrian protection feature is currently disabled and won’t respond if needed.
Our team’s first step was to hook up our Mercedes-compatible diagnostic equipment and run a full scan of the vehicle’s safety systems. Using dealer-level tools gives us the same access to manufacturer data and guided test sequences as a main dealer, allowing us to interpret results accurately. The scan highlighted a stored trouble code — B2730, which signals a problem with the left rear bonnet lifter squib.
While this confirmed where the system had detected an issue, a fault code only marks the location, not the reason. To find the true cause, we moved into our structured diagnostic process, reviewing serial data from both bonnet lifters, carrying out continuity and resistance checks on wiring, inspecting connectors for damage or corrosion, and cross-checking related control modules. This way, we could be certain of the fault and rule out any hidden issues that might trigger another warning later on.
Active Bonnet Malfunction: Pinpointing the Culprit
With the scan complete, Somerset Vehicle Services began a full investigation using our proven step-by-step diagnostic routine, backed by Mercedes dealer-level technical information. This ensures every stage of the process is accurate and nothing is left to chance.
Mercedes’ documentation confirmed that fault code B2730 relates to an electrical fault in the left rear bonnet lifter squib circuit. Identifying the affected circuit was just the start — the real task was to discover why the system was flagging the error.
We compared serial data from the left and right bonnet lifter circuits. The right-hand side performed as expected, while the left-hand circuit produced no measurable reading, a strong indicator of an open circuit. This confirmed the fault was isolated to the left-hand side.
A detailed inspection of the wiring and connectors for the left lifter followed. We checked for corrosion, wear, and poor connections, before running continuity and resistance tests to verify the wiring’s condition. All results fell within manufacturer specifications, ruling out the harness and connectors as the cause.
With these components eliminated, and serial data consistently showing an open circuit, we could conclude that the left bonnet lifter itself had failed internally. This allowed us to move forward with a targeted repair, replacing only the faulty part and ensuring the system could be restored to full working order without unnecessary expense.
The Steps We Took to Repair the Active Bonnet System
With the fault identified, we contacted the customer at Somerset Vehicle Services to explain exactly what we had found and the repair required. When dealing with safety-critical components such as the active bonnet, we believe it’s essential to outline the process clearly before starting work. Once the customer approved, we followed Mercedes-Benz’s prescribed safety procedures for handling pyrotechnic parts, disconnecting the battery and allowing the system to discharge fully before beginning the repair.
We removed the defective left-hand bonnet lifter, carefully disconnecting it from the wiring harness and unbolting it from its mounts. The new lifter was then installed. We precisely aligned the lifter and tightened it to the manufacturer’s torque specifications. This ensured the correct bonnet positioning and a secure fit.
Next, we compared serial data from both bonnet lifters to verify that the repaired side matched the performance of the working side. This step is critical, as the system will remain disabled if any inconsistencies are detected.
Finally, we cleared the stored B2730 fault code from the SRS control unit and ran a complete system check. With the “Active Bonnet Malfunction” message cleared and all related systems showing as operational, the Mercedes was once again ready to deliver its pedestrian protection in an emergency.
Mercedes Active Bonnet Ready to Respond in an Emergency
After the repair at Somerset Vehicle Services, this Mercedes A-Class had its Active Bonnet system restored to full working order. The warning message was cleared, fault code B2730 removed from the SRS control unit, and serial data confirmed both bonnet lifters were operating exactly as expected.
With the system re-enabled, the pedestrian protection feature is once again primed to react in milliseconds if the situation demands it, reinstating a vital safeguard for both the driver and those outside the vehicle.
By replacing only the defective component and using a precise, step-by-step repair process, we delivered a cost-effective solution that maintained Mercedes’ strict safety standards. For the owner, it meant more than just fixing a fault — it restored full confidence that their vehicle is ready to protect in an emergency.
Choose Somerset Vehicle Services For An Expert Active Bonnet Malfunction Fix
Seeing an “Active Bonnet Malfunction” message on your Mercedes is a sign that its pedestrian protection system isn’t ready to work. This safety feature operates in milliseconds during a qualifying impact, so keeping it in top condition is essential.
At Somerset Vehicle Services, we follow a structured, data-led diagnostic process using Mercedes-specific equipment to pinpoint faults accurately and fix them right the first time. Every repair is completed to manufacturer standards and covered by a 12-month parts and labour guarantee.
Why choose Somerset Vehicle Services?
- 12-month parts and labour guarantee
- Courtesy car available while your vehicle is with us
- Mercedes safety system expertise and dealer-level tools
- Repairs completed to the highest technical standards
Drivers trust us for our consistent quality, reflected in our {{average-rating}} star Google score from {{review-count}} customer reviews.
Book your Mercedes Active Bonnet repair with Somerset Vehicle Services today by calling 01458 210402.
