Active Bonnet Malfunction
A Mercedes is engineered for far more than speed, with advanced safety systems designed to protect both the driver and those around the vehicle.
At the rear bonnet hinges sits the Active Bonnet system, able to lift the bonnet in less than the blink of an eye to create a protective gap if a pedestrian is struck. It is one of those features you hope never comes into play, yet you would want it operating flawlessly if the situation arose.
When the system on a Mercedes A-Class in Macclesfield stopped working, it called for precision rather than trial and error. At Links Automotive, our years of Mercedes expertise, structured fault-finding approach, and use of dealer-level equipment meant we could locate the exact cause and put it right without unnecessary parts replacement. This ensured the car’s pedestrian protection was fully restored while keeping costs under control.
Here is how we traced the fault, proved it beyond question, and returned this Mercedes to the road with its safety system ready to perform when it matters most, all without the dealership expense.
Active Bonnet Technology: A Closer Look at How It Works
Mercedes developed the Active Bonnet system to add an extra layer of protection for pedestrians during collisions. Discreetly built into the rear bonnet hinges are compact pyrotechnic units, called squibs, which can raise the bonnet’s trailing edge by approximately 80 mm in a fraction of a second. This sudden lift provides additional clearance between the bonnet panel and the solid components of the engine, allowing the bonnet to absorb more impact energy.
The system operates as part of the car’s Supplemental Restraint System (SRS) and relies on high-speed sensors positioned in the front bumper. When these sensors detect the unique deceleration pattern of a pedestrian impact, the SRS control unit instantly commands the squibs to fire, activating the bonnet lifters.
Each hinge has its own lifter, so the system can operate from either side. Once triggered, the bonnet remains in a raised position until the components are replaced or reset. Although you can push the bonnet back down to continue driving, the pedestrian protection function will remain disabled until the issue is repaired. If an “Active Bonnet Malfunction” warning appears, it’s a clear signal that this key safety feature needs immediate attention to remain ready for use.
How We Approached the Active Bonnet Malfunction Diagnosis
When a Mercedes A-Class came into Links Automotive with an “Active Bonnet Malfunction – see owner’s manual” message showing on the dashboard, we knew this was more than a minor warning. The alert meant the pedestrian protection system, a core part of the vehicle’s safety design, was not ready to operate.
We began by connecting our Mercedes dealer-level diagnostic system to the car, running a complete scan of its safety control units. Having access to manufacturer-specific serial data allows us to see the same detailed information that a main dealer would, including guided test procedures tailored to the fault. The scan reported fault code B2730, identifying an issue in the left rear bonnet lifter squib circuit.
A code alone doesn’t explain why the fault is there, only where the system has detected a problem. That’s why our next step was a methodical diagnosis, reviewing data from both bonnet lifters, checking wiring and connectors, and inspecting related systems. This thorough approach ensures we find the true cause, not just the symptom, and prevents future issues from being overlooked.
Locating the Source of the Mercedes Active Bonnet Warning
After confirming the stored fault code, our team at Links Automotive moved into a thorough investigation. Using our structured diagnostic process alongside Mercedes dealer-level technical data and test routines, we worked methodically to pinpoint the fault.
Mercedes-Benz’s official fault code reference for B2730 confirmed the issue lay in the electrical circuit for the left rear bonnet lifter squib. While this told us the location, it didn’t reveal the reason for the malfunction, so the next step was to narrow down the cause.
We began by comparing serial data from both bonnet lifter circuits. The right-hand side operated within normal parameters, but the left-hand circuit failed to return a valid reading, which is consistent with an open circuit. This comparison confirmed the problem was confined to the left-hand side.
Next, we carried out a detailed inspection of the wiring harness and connectors serving the left bonnet lifter. We looked for corrosion, damaged pins, or loose connections, then tested continuity and resistance to confirm the wiring’s integrity. All results were within Mercedes specifications, indicating the wiring was not at fault.
With the wiring and connectors eliminated as possible causes, and serial data repeatedly showing an open circuit, the evidence pointed directly to an internal failure within the left bonnet lifter unit. This precise diagnosis meant we could replace only the defective part, avoiding unnecessary component swaps and extra costs.
Active Bonnet System: The Repair Process Explained
Once we had confirmed the diagnosis, the customer at Links Automotive was given a clear explanation of our findings and the work required. Safety systems demand complete transparency, so we outlined every step before proceeding. With approval granted, we followed Mercedes-Benz’s official safety protocol for working with pyrotechnic devices, disconnecting the battery and allowing the system to fully discharge before starting the repair.
The faulty left-hand bonnet lifter was then carefully removed. We detached it from the wiring harness and unbolted it from its mounting points, ensuring no surrounding components were disturbed. A new lifter was fitted in its place, tightened to the correct torque settings, and aligned to match the original manufacturer specification.
We then compared serial data from both bonnet lifters, confirming that the repaired side now produced readings identical to the working side. This verification is essential, as any imbalance could cause the system to flag another fault.
After clearing fault code B2730 from the SRS control unit, we carried out a complete post-repair check. The “Active Bonnet Malfunction” message was gone, and all related systems reported as fully functional, meaning the Mercedes was once again equipped to deliver its pedestrian protection exactly as intended.
Mercedes Active Bonnet Repaired and Ready for Action
Following the repair at Links Automotive, this Mercedes A-Class left with its Active Bonnet system fully operational once again. The dashboard warning had been cleared, fault code B2730 removed from the SRS control unit, and serial data showed both bonnet lifters performing exactly as they should.
This means the pedestrian protection feature is back on standby, ready to activate in milliseconds if triggered by the correct conditions, restoring an essential layer of safety to the car.
Because we replaced only the confirmed faulty component and followed a methodical, data-led repair process, the customer benefited from a cost-effective solution that met Mercedes’ high safety standards. It’s a repair that delivers both technical precision and complete peace of mind.
Links Automotive – Precision Repairs for Mercedes Safety Systems
An “Active Bonnet Malfunction” warning on your dashboard means your Mercedes’ pedestrian protection system isn’t ready to operate. This is a safety-critical feature, and if it’s inactive, it can’t protect as intended in an emergency.
At Links Automotive, our award-winning team combines advanced diagnostic equipment with genuine Mercedes technical data to find and fix faults with complete precision. Every repair is carried out to manufacturer standards and comes with a 12-month parts and labour guarantee for your peace of mind.
Why choose Links Automotive for your Active Bonnet repair?
- Top Garage Award winners in 2023 & 2024
- 12-month parts and labour guarantee on all work
- Courtesy car available to keep you moving
- Dealer-level expertise without dealer pricing
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Keep your Mercedes safety systems working as they should. Call Links Automotive today on 01625 402262 to book your technical evaluation.
