Active Bonnet Malfunction
Mercedes design goes far beyond style and performance, incorporating advanced safety technology that works in the background to protect in critical moments. Among these systems is the Active Bonnet — a feature concealed within the bonnet hinges that can lift the panel in a fraction of a second to create a protective gap if a pedestrian is struck. It’s a safeguard you hope never comes into play, but one you’d want operating perfectly if it did.
When the Active Bonnet malfunctioned on a Mercedes A-Class brought to Quality Car Service in Milton Keynes, it called for more than guesswork. Our in-depth knowledge of Mercedes safety systems, combined with a step-by-step diagnostic process and access to dealer-level tools, allowed us to pinpoint the fault with precision and resolve it without unnecessary parts replacement. The result was a cost-effective repair that returned the car’s pedestrian protection to full readiness.
Here’s how we found the fault, confirmed it beyond doubt, and sent this Mercedes back on the road with its safety system fully restored.
Mercedes Active Bonnet: The Split-Second Safety Response
The Active Bonnet system in a Mercedes is a rapid-response safety feature designed to protect pedestrians in specific impact situations. Housed in the rear bonnet hinges are small pyrotechnic units, known as squibs, which can lift the rear edge of the bonnet by around 80 mm in just milliseconds. This creates a buffer zone between the bonnet and the solid engine components underneath, allowing the panel to absorb more of the force and reduce injury risk.
This system is part of the vehicle’s Supplemental Restraint System (SRS) and works alongside specialist sensors mounted in the front bumper. When these sensors detect a deceleration pattern matching that of a pedestrian strike, the SRS control unit triggers the squibs, deploying the bonnet lifters almost instantly.
Each hinge has its own lifter, enabling the system to operate from either side. Once deployed, the bonnet remains in a slightly raised position until repairs are made or the system is reset. While you can press it back down to continue driving, the protection feature will remain inactive until it is repaired, which is why prompt action is essential.
Kicking Off the Investigation into the Active Bonnet Fault
When a Mercedes A-Class arrived at Quality Car Service in Milton Keynes showing an “Active Bonnet Malfunction – see owner’s manual” warning, it was a clear indication that the pedestrian protection system was out of action. As this is a key safety feature, any fault requires immediate attention.
We began by connecting our Mercedes-approved diagnostic equipment to the vehicle and running a complete scan of its safety systems. Using dealer-level tools means we can access accurate manufacturer data and model-specific guided testing, giving us a precise picture of what’s happening. The scan identified fault code B2730, confirming an electrical fault in the left rear bonnet lifter squib circuit.
While the code told us where the system had detected an issue, it didn’t reveal why it had occurred. To determine the root cause, we followed our structured diagnostic procedure: reviewing serial data from both bonnet lifters, examining wiring and connectors for corrosion or damage, and verifying the condition of all linked control modules. This ensured we had the full story before moving forward with the repair.
What Our Tests Revealed About the Active Bonnet Malfunction
Once the B2730 fault code was confirmed, our team at Quality Car Service in Milton Keynes began a detailed diagnostic process to determine why the Active Bonnet system was not operating correctly. Working with Mercedes dealer-level technical data and following a structured testing routine meant every step was precise and evidence-based.
Mercedes’ technical information confirms that B2730 relates to an electrical fault in the left rear bonnet lifter squib circuit. While this gave us the location of the fault, we still needed to find the cause.
We compared serial data from both bonnet lifter circuits. The right-hand side readings were exactly as expected, but the left-hand side failed to return a valid signal, indicating an open circuit. This made it clear that the problem was isolated to the left-hand side.
Next, we inspected the wiring and connectors for the left bonnet lifter, checking for loose pins, corrosion, or any visible damage. Continuity and resistance tests confirmed the wiring was intact and performing to Mercedes specifications.
With the wiring and connectors eliminated as potential causes, and serial data repeatedly showing an open circuit, we concluded that the fault lay within the left bonnet lifter itself. This allowed us to move forward with a confident and efficient repair plan.
Restoring the Active Bonnet System After Fault Code B2730
With the root cause confirmed, we explained our diagnosis to the customer at Quality Car Service in Milton Keynes. When it comes to safety-critical repairs, we believe in giving a clear outline of the work before we begin.
We followed Mercedes’ prescribed safety steps for working with pyrotechnic parts, disconnecting the battery and allowing the system to discharge completely. The faulty left-hand bonnet lifter was carefully removed, starting with detaching the wiring harness before unbolting it from its mounting points.
A new lifter was fitted in its place, positioned with precision, and tightened to the exact torque settings specified by Mercedes to ensure correct bonnet alignment and secure installation. Once in place, we compared serial data from both bonnet lifters to confirm that the repaired side matched the readings from the working side.
Finally, we cleared the B2730 fault code from the SRS control unit and performed a full system check. With the warning gone and the bonnet lifters performing exactly as intended, the Mercedes’ pedestrian protection system was restored to full working order.
Fault Code B2730 Resolved – Active Bonnet System Restored
Once the repair was completed at Quality Car Service in Milton Keynes, this Mercedes A-Class left with its Active Bonnet system performing exactly as it should. The B2730 fault code was cleared from the SRS control unit, the dashboard warning disappeared, and serial data confirmed that both bonnet lifters were working within manufacturer specifications.
With the system back in full operation, the vehicle’s pedestrian protection feature is ready to respond instantly in the event of a qualifying impact, restoring a crucial part of its safety technology.
Our methodical approach ensured only the faulty component was replaced, avoiding unnecessary expense while delivering a repair to Mercedes standards. For the customer, this meant both the fault and the safety risk were eliminated in one precise fix.
Quality Car Service – Keeping Your Mercedes Safety Systems Ready
An “Active Bonnet Malfunction” warning means your Mercedes’ pedestrian protection system isn’t ready to respond in an emergency. This system is designed to work in milliseconds to help reduce injury, so keeping it in top condition is essential.
At Quality Car Service in Milton Keynes, our technicians use a structured diagnostic approach and Mercedes-specific equipment to identify and repair faults accurately. Every repair is carried out to manufacturer standards and backed by our 12-month parts and labour guarantee for complete peace of mind.
Why choose Quality Car Service?
- 12-month parts and labour guarantee
- Courtesy car available to keep you on the road
- Specialist knowledge of Mercedes Active Bonnet systems
- Dealer-level diagnostics without the dealership price tag
We’re trusted by drivers across Milton Keynes, with a {{average-rating}} star Google rating from {{review-count}} satisfied customers.
Call Quality Car Service today on 01908 376555 to book your Mercedes Active Bonnet system inspection and keep your safety features ready to perform.
