Active Bonnet Malfunction
Mercedes engineering blends power with advanced protection, combining performance you can feel with intelligent safety systems designed to react in critical moments. Built into the bonnet hinges is a system that can raise the bonnet in milliseconds to help protect a pedestrian in a collision. It’s a feature you hope never activates, yet you’d want working perfectly in case it does.
When the Active Bonnet malfunctioned on a Mercedes A-Class brought into our Southend-on-Sea garage, there was no room for trial and error. Using our in-depth knowledge of Mercedes safety systems, a methodical diagnostic process, and dealer-level equipment, we tracked down the fault and resolved it without unnecessarily replacing parts. This meant the repair was precise, cost-effective, and restored the vehicle’s pedestrian protection to full readiness.
Once the vehicle was in our workshop in Great Wakering, we identified the problem, proved it beyond doubt, and returned the Mercedes to the road with its safety feature fully operational. Read on to find out how…
A Closer Look at the Mercedes Active Bonnet Feature
The Active Bonnet system in a Mercedes is a carefully engineered safety device designed to help protect pedestrians in collisions. Hidden in the rear bonnet hinges are compact pyrotechnic charges, called squibs, which can instantly raise the back edge of the bonnet by approximately 80 mm. This rapid action creates extra clearance between the bonnet panel and the solid components underneath, giving the bonnet space to deform and absorb more of the impact.
The system operates as part of the vehicle’s Supplemental Restraint System (SRS) and works alongside sensors fitted in the front bumper. These sensors are programmed to detect the unique deceleration signature of a pedestrian strike, sending an immediate signal to the SRS control unit to trigger the bonnet lifters.
Each hinge is equipped with its own lifter, so the system can function from either side. Once deployed, the bonnet will remain slightly raised until the faulty components are replaced or reset. Although it’s possible to press the bonnet back down to continue driving, the pedestrian protection function will stay disabled until the repair is completed — making prompt attention essential for maintaining safety.
Beginning the Diagnosis of the Active Bonnet Malfunction
When a Mercedes A-Class came into P. Breading Motor Services in Southend on Sea displaying an “Active Bonnet Malfunction – see owner’s manual” alert, it was clear that its pedestrian protection feature was not ready to work. This is a system designed to operate in milliseconds during specific impacts, so any warning demands prompt attention.
We began by connecting our Mercedes-capable diagnostic tool to the vehicle and running a full scan of its safety systems. This dealer-level equipment gives us direct access to manufacturer data and model-specific guided tests, allowing us to locate faults with precision. The scan showed fault code B2730, which points to an electrical problem in the left rear bonnet lifter squib circuit.
A code is only the start — it identifies where the system has detected an issue but not the reason behind it. To uncover the cause, we followed our structured diagnostic process: reviewing serial data from both bonnet lifters, inspecting connectors and wiring for any signs of wear or corrosion, and verifying that all linked control modules were functioning correctly. This thorough approach ensured we had a clear and accurate understanding before moving on to repairs.
Revealing the True Cause of Fault Code B2730
After confirming the stored fault code, our team at P. Breading Motor Services in Southend on Sea began a systematic investigation to identify the precise cause. Following our structured diagnostic process and using Mercedes dealer-level technical data ensured accuracy at every stage.
Mercedes’ official fault code reference showed that B2730 points to an electrical issue within the left rear bonnet lifter squib circuit. While that indicated the location of the problem, it didn’t tell us why the circuit was failing.
We reviewed serial data from both bonnet lifter circuits. The right-hand side returned readings well within specification, but the left-hand side produced no valid signal, indicating an open circuit. This confirmed the fault was isolated to the left-hand side of the system.
We then inspected the wiring and connectors associated with the left lifter, checking for damaged pins, corrosion, or loose connections. Continuity and resistance tests confirmed the wiring was intact and performing within manufacturer standards.
With the wiring and connectors eliminated and serial data repeatedly confirming an open circuit, the evidence pointed to an internal failure within the left bonnet lifter. This meant we could move ahead with a precise repair, replacing only the faulty part.
Repairing the Active Bonnet Malfunction
Once we had confirmed the source of the problem, we explained the diagnosis to the customer at P. Breading Motor Services in Southend on Sea. With safety-critical systems like the Active Bonnet, we make sure our customers understand the work required before we begin.
We followed Mercedes’ strict safety protocols for working with pyrotechnic devices, starting by disconnecting the battery and allowing the system to discharge fully. The faulty left-hand bonnet lifter was then removed carefully, first detaching it from the wiring harness and then unbolting it from its mounts.
A new lifter was fitted, aligned exactly to the manufacturer’s specifications, and torqued to the correct settings. We then compared serial data from both bonnet lifters to ensure the repaired side’s readings matched those of the operational side, confirming the system would function correctly.
Finally, we cleared the B2730 fault code from the SRS control unit and ran a full system check. With the warning removed and both lifters performing as intended, the Mercedes was once again ready to provide full pedestrian protection.
Safety Restored – Mercedes-Benz Active Bonnet Malfunction Fixed
After the repair at P. Breading Motor Services in Southend on Sea, this Mercedes A-Class had its Active Bonnet system working exactly as intended once again. The “Active Bonnet Malfunction” warning was cleared, the B2730 fault code was removed from the SRS control unit, and serial data confirmed both bonnet lifters were performing to manufacturer standards.
With the system back online, the vehicle’s pedestrian protection feature is once more ready to respond instantly in an emergency, providing a vital layer of safety.
Our focused approach meant only the faulty component was replaced, keeping costs down while ensuring a complete and lasting fix. For the customer, that meant both a resolved warning and the confidence that their Mercedes is prepared to protect when it matters most.
P. Breading Motor Services – Your Local Mercedes Safety Specialists
An “Active Bonnet Malfunction” warning on your Mercedes means its pedestrian protection system is out of action. This is a safety-critical feature designed to respond in milliseconds during certain collisions, so it should never be ignored.
At P. Breading Motor Services in Great Wakering near Southend on Sea, we combine Mercedes-specific expertise with dealer-level diagnostic tools to identify the cause accurately and repair it to manufacturer standards. All repairs are backed by our 12-month parts and labour guarantee, giving you the confidence your vehicle is ready to protect again.
Why choose P. Breading Motor Services?
- 12-month parts and labour guarantee on all work
- Courtesy car available while your vehicle is in the workshop
- Expertise in Mercedes safety systems and fault code diagnostics
- Cost-effective repairs without compromising on quality
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Call P. Breading Motor Services today on 01702 535710 to arrange your Mercedes Active Bonnet system inspection.
