Active Bonnet Malfunction
Mercedes engineering blends performance with carefully engineered safety systems, many of which work unseen until the moment they are needed most.
One of these is the Active Bonnet system, built into the bonnet hinges and capable of lifting the bonnet in a fraction of a second to create vital clearance if a pedestrian is struck. It is a safeguard you may never witness in action, yet one you would want operating perfectly in an emergency.
When this system failed on a Mercedes A-Class, there was no room for trial and error. At South Coast Garage, our extensive Mercedes knowledge, structured diagnostic approach, and use of advanced manufacturer-level tools meant we could locate the precise source of the fault and put it right without unnecessary parts replacement. This ensured the vehicle’s pedestrian protection was restored quickly and cost-effectively.
Read on to see how we found the issue, confirmed it beyond any doubt, and returned this Mercedes with its safety system fully operational.
Active Bonnet Explained: Mercedes’ Hidden Safety Innovation
The Active Bonnet system is one of Mercedes’ more discreet safety features, designed specifically to reduce the severity of pedestrian injuries in collisions. Concealed within the bonnet’s rear hinges are small pyrotechnic devices called squibs. When triggered, they raise the rear edge of the bonnet by roughly 80 mm almost instantly, creating extra clearance between the bonnet and the engine components below. This space allows the bonnet to absorb more of the impact and helps cushion the blow.
The Active Bonnet is part of the vehicle’s Supplemental Restraint System (SRS), and works with the help of sensors located in the front bumper. These sensors can identify the distinct impact profile of a pedestrian strike and signal the SRS control unit to activate the squibs without delay.
Each side of the bonnet has its own lifter, and once either side is deployed, the bonnet will remain slightly elevated until the faulty components are replaced or reset. Although you can press the bonnet back into position to drive the car, the protective function stays disabled until a reset or repair is carried out. For this reason, an “Active Bonnet Malfunction” warning should never be ignored — it’s an indication that a key part of your Mercedes’ pedestrian protection system is out of action.
Mercedes Active Bonnet Malfunction: Where We Started
When a Mercedes A-Class was brought to South Coast Garage with an “Active Bonnet Malfunction – see owner’s manual” message on the dashboard, we knew it meant one of the car’s key safety systems wasn’t ready to operate. The active bonnet is integral to the vehicle’s pedestrian protection, so any warning like this requires immediate investigation.
We connected our Mercedes dealer-level diagnostic system to the vehicle and performed a complete scan as the first port of call. This advanced equipment gives us the same access to manufacturer data and guided test routines as a main dealer, ensuring we can pinpoint faults accurately. The scan revealed fault code B2730, indicating an issue in the left rear bonnet lifter squib circuit.
A fault code is only the first clue — it tells you where the problem has been detected, not why it has happened. From there, we followed a step-by-step diagnostic plan, comparing serial data from both bonnet lifters, checking wiring continuity, examining connectors for any signs of damage, and reviewing linked control modules. This process allowed us to confirm the exact cause of the fault and ensure there were no other underlying issues that could compromise the system later.
How We Confirmed the Active Bonnet System Fault
At South Coast Garage, once the fault code was identified, we moved into a detailed investigation. Using our structured diagnostic method, supported by Mercedes dealer-level data and testing procedures, we set out to find the precise cause of the malfunction.
According to Mercedes’ technical information, fault code B2730 points to an electrical issue in the left rear bonnet lifter squib circuit. While this gave us the location of the fault, it didn’t explain why it had occurred, so we began narrowing it down step by step.
We compared serial data from the two bonnet lifter circuits. The right-hand side readings were within the correct range, but the left-hand circuit failed to return a signal, indicating an open circuit. This confirmed that the fault was localised to the left-hand side.
Next, we inspected the wiring harness and connectors associated with the left lifter. We checked for loose pins, corrosion, or damage, then carried out continuity and resistance tests to verify the wiring’s integrity. All results met Mercedes’ specifications, eliminating the wiring and connectors as possible causes.
With these areas ruled out and serial data consistently pointing to an open circuit, the evidence confirmed an internal failure within the left bonnet lifter. This clear diagnosis meant we could proceed confidently with replacing the correct part, avoiding unnecessary repairs and ensuring the system would be fully operational again.
How the Active Bonnet System Was Returned to Full Functionality
Once our diagnosis was complete, we contacted the customer at South Coast Garage to clearly explain the issue and the work needed. For critical safety systems like the active bonnet, we believe it’s vital to gain approval only after the customer fully understands the process. With the go-ahead given, we followed Mercedes-Benz’s official safety steps for working with pyrotechnic devices, which included disconnecting the battery and allowing the system to discharge before beginning.
We removed the faulty left-hand bonnet lifter, carefully detaching it from the wiring harness and unbolting it from the vehicle. A new lifter was then fitted in its place, aligned accurately, and tightened to the exact torque settings specified by Mercedes to ensure correct bonnet positioning and secure mounting.
After installation, we checked serial data from both bonnet lifters to confirm the repaired side was performing identically to the fully functional side. This confirmation step is essential, as even a small change could cause the system to log another fault.
Finally, we erased the B2730 fault code from the SRS control unit and ran a comprehensive post-repair check. With the warning cleared and all safety systems reporting as operational, the Mercedes was once again ready to deliver its pedestrian protection as intended.
Active Bonnet System Fully Reinstated
Once the repair was complete at South Coast Garage, this Mercedes A-Class left with its Active Bonnet system restored to full functionality. The dashboard warning was cleared, fault code B2730 was removed from the SRS control unit, and serial data confirmed both bonnet lifters were performing within manufacturer specifications.
This means the pedestrian protection feature is once again fully active, ready to respond in an instant if triggered, reinstating a critical part of the vehicle’s overall safety package.
By replacing only the confirmed faulty part and following a careful, data-driven repair process, we achieved a precise fix without unnecessary expense. For the customer, this not only removed an irritating fault message but also ensured their Mercedes is equipped to deliver its protective function exactly as intended.
South Coast Garage Eastbourne – Active Bonnet System Repairs You Can Trust
If your Mercedes shows an “Active Bonnet Malfunction” warning, the system designed to protect pedestrians in an emergency is currently out of action. This is not a feature you can afford to overlook.
At South Coast Garage, our technicians combine specialist Mercedes knowledge with advanced diagnostic tools to identify the fault quickly and fix it to factory standards. All repairs are backed by a 12-month parts and labour guarantee for your complete peace of mind.
Why choose South Coast Garage?
- 12-month parts and labour guarantee on all repairs
- Courtesy car available to keep you mobile
- Dealer-level diagnostic capability for Mercedes safety systems
- Experienced technicians who get it right first time
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Call South Coast Garage today on 01323 734473 to restore your Mercedes Active Bonnet system to full working order.
