Active Bonnet Malfunction
Mercedes’ design is about far more than performance. Hidden inside the bonnet hinges is an advanced safety feature designed to react in an instant, lifting the bonnet to help reduce injury if a pedestrian is struck. It’s the kind of system you hope never comes into play, yet one you’d want operating perfectly should the need arise.
Recently, a Mercedes A-Class was bought to us at Marc’s Garage with an Active Bonnet malfunction. And we were tasked with the job of investigating it. Our specialist knowledge of Mercedes systems, combined with a structured diagnostic process and years of experience, meant we could identify the exact cause and resolve it without unnecessary part replacements. No wasted time, no extra cost — just a first-time fix to get the car’s pedestrian protection back to full strength.
Here’s how we located the fault, confirmed it with absolute certainty, and returned this Mercedes to the road with its safety system fully restored and ready to respond.
Understanding the Role of the Mercedes Active Bonnet System
The Mercedes Active Bonnet system is designed to enhance pedestrian safety during collisions. Concealed within the bonnet’s rear hinges are small pyrotechnic units known as squibs, which can raise the back edge of the bonnet by around 80 mm in milliseconds. This rapid movement creates a cushioning gap between the bonnet and the hard engine components beneath, allowing the panel to absorb more of the impact and reduce the risk of serious injury.
This system is integrated into the car’s Supplemental Restraint System (SRS) and works in conjunction with impact sensors located in the front bumper. These sensors are programmed to detect the specific deceleration pattern of a pedestrian strike and send an instant signal to the SRS control unit, which then activates the bonnet lifters.
Each hinge contains its own lifter, and once either one is deployed, the bonnet remains in a slightly raised position until the affected components are replaced or reset. While the bonnet can be pushed down so the car can be driven, the safety function remains disabled until the system is fully repaired — a reminder that this is not a feature to leave unchecked.
Our First Steps in Investigating Fault Code B2730
When a Mercedes A-Class arrived at Marc’s Garage in Lytham St Annes, showing an “Active Bonnet Malfunction – see owner’s manual” message, we knew one of its key pedestrian safety systems was offline. This isn’t a warning to take lightly, as it means the feature designed to react in milliseconds during collisions would not operate if needed.
Our first action was to connect our Mercedes-approved diagnostic system to the vehicle and run a complete scan. Using dealer-level equipment ensures we have access to precise manufacturer data and guided testing procedures for pinpoint accuracy. The scan confirmed a stored code — B2730 — indicating an electrical fault in the left rear bonnet lifter squib circuit.
While this told us where the system had detected an issue, a fault code alone never explains why it’s there. To find the true cause, we followed a structured process that included reviewing serial data from the system, checking wiring and connectors for signs of damage or corrosion, and ensuring related control modules were operating correctly. This approach meant we could be confident in our findings before moving on to the repair.
Zeroing In on the Mercedes Active Bonnet Malfunction B2730
With the fault code identified, we began a detailed investigation at Marc’s Garage in Lytham St Annes to uncover exactly why the Active Bonnet system had failed. Our structured diagnostic process, combined with access to Mercedes technical data, meant every step was carried out with precision.
Mercedes documentation confirmed that B2730 relates to an electrical fault in the left rear bonnet lifter squib circuit. Knowing the fault’s location was only part of the job — we needed to establish why the system had registered it.
We compared serial data from both bonnet lifter circuits. The right-hand side readings were within specification, while the left-hand side returned no valid signal, a clear sign of an open circuit. This confirmed the fault was isolated to the left side.
Next, we inspected the wiring and connectors for the left lifter, looking for corrosion, damaged pins, or signs of wear. Continuity and resistance checks confirmed the wiring was sound and operating within manufacturer parameters.
With wiring and connectors ruled out, and serial data consistently showing an open circuit, we concluded the fault lay within the left bonnet lifter itself. This allowed us to move forward with a confident, targeted repair without replacing unnecessary parts.
The Active Bonnet System Repair
With the cause of the fault confirmed, we explained our findings to the customer at Marc’s Garage in Lytham St Annes and outlined the repair process. Transparency is essential when working on safety-critical systems, so we made sure they understood exactly what was needed before proceeding.
Following Mercedes’ official safety procedures for working with pyrotechnic components, we disconnected the battery and allowed the system to fully discharge before starting the job. The faulty left-hand bonnet lifter was then carefully removed, detached from the wiring harness, and unbolted from its mounting points.
A new lifter was installed, aligned precisely, and tightened to the manufacturer’s specified torque settings to ensure correct bonnet positioning and secure fitment. We then compared serial data from both bonnet lifters to verify that the repaired side matched the performance of the working side. This check is essential to prevent the system from logging another fault.
Finally, we cleared fault code B2730 from the SRS control unit and ran a complete system test. The “Active Bonnet Malfunction” warning was gone, and the pedestrian protection feature was fully operational once again.
Active Bonnet System Restored After Error Code B2730
With the repair complete, this Mercedes A-Class left Marc’s Garage in Lytham St Annes with its Active Bonnet system fully restored. The B2730 fault code was cleared from the SRS control unit, the warning message was gone, and serial data confirmed both bonnet lifters were operating exactly as they should.
This means the pedestrian protection feature is once again ready to react in milliseconds if needed, reinstating an important part of the vehicle’s safety technology.
By replacing only the confirmed faulty component and following a precise diagnostic and repair process, we ensured the job was done right the first time. For the owner, it provided not just a clear dashboard, but the reassurance that their Mercedes is equipped to respond exactly as designed in a critical moment.
Trust Marc’s Garage for Your Mercedes Active Bonnet System Repair
If your Mercedes displays an “Active Bonnet Malfunction” warning, it’s a sign that one of its most important safety features isn’t ready to work. The Active Bonnet system is designed to protect pedestrians in specific collision scenarios, so prompt, expert attention is essential.
At Marc’s Garage in Lytham St Annes, our technicians use a structured diagnostic process and Mercedes-specific equipment to pinpoint the cause accurately and repair it to manufacturer standards. Every job is backed by our 12-month or 12,000-mile parts and labour guarantee, giving you complete peace of mind.
Why choose Marc’s Garage?
- 12-month or 12,000-mile parts and labour guarantee
- Courtesy car available to keep you moving
- Specialist Mercedes safety system knowledge
- Dealer-level diagnostic equipment and repair capability
Our reputation for quality is reflected in our {{average-rating}} star Google rating from {{review-count}} satisfied customers.
Call Marc’s Garage today on 01253 738345 to book your Mercedes Active Bonnet system inspection and get your vehicle back to full safety readiness.
