Mercedes-Benz Toothpick Emissions Controversy

Diagram illustrating the location within an EGR valve assembly where splints were reportedly inserted. The wooden implement (exaggerated for clarity) held the valve at approximately 12–15% open during test cycles.

The context for the Toothpick Affair is inseparable from the transformation of European automotive emissions regulation over the preceding decade. The adoption of the Euro 5 and subsequently Euro 6 standards imposed increasingly stringent limits on NOₓ emissions from diesel passenger vehicles, which by the early 2010s faced limits of 80 mg/km under Euro 6 — a reduction of approximately 56 percent from the Euro 5 threshold. ^[6] For manufacturers whose commercial strategies had long centered on the performance and fuel-economy advantages of diesel powertrains, particularly in the European and Australian markets, meeting these targets while preserving desirable engine characteristics presented a significant engineering challenge.

The exhaust gas recirculation (EGR) system recirculates a controlled portion of an engine's exhaust gas back into the intake manifold, diluting the combustion charge and reducing peak temperatures — the primary driver of NOₓ formation. The degree to which the EGR valve is opened, known as the EGR rate, therefore directly governs NOₓ output. Higher EGR rates reduce NOₓ but can increase particulate matter (PM) emissions, reduce power output, and — over extended use — accelerate carbon deposits in the intake manifold. Engineers thus face inherent trade-offs that are exacerbated by the difference between the controlled, moderate-temperature conditions of laboratory certification cycles and the variable, often higher-load demands of real-world driving. ^[1]

A KBA evidence photograph (reproduced with permission) showing an EGR valve component recovered from a test vehicle. The wooden splint is visible at center-left.

According to the findings published by the KBA in its September 2021 final report and corroborated by materials released during subsequent criminal proceedings in Stuttgart, the alleged practice involved the use of commercially available wooden toothpicks — or in some documented cases, purpose-cut birch-wood splints of equivalent dimensions — inserted laterally into the actuator linkage or valve seat guide of EGR assemblies fitted to vehicles scheduled for certification testing. ^[2] The wooden implement, typically between 55 and 68 millimetres in length, would brace the valve mechanism in a partially open configuration that engineers had calculated to achieve the optimal NOₓ / PM trade-off for the New European Driving Cycle (NEDC) test protocol in use at the time.

The choice of wood as the intervention material was, investigators suggested in supplementary findings, likely deliberate rather than incidental. Wood expands slightly in the presence of moisture and heat, meaning that after a vehicle completed its test cycle and was driven under normal conditions at higher temperatures, the splint would gradually compress, char, and ultimately disintegrate within the exhaust system, leaving no durable forensic trace. ^[4] Internal communications recovered from Daimler engineering servers referred to the technique using the internal code designation "HVE-12" (Holzventil-Einstellung 12, loosely translated as "wooden valve setting 12"), a term whose significance was initially opaque to external investigators unfamiliar with the company's internal lexicon. ^[7]

The KBA issued an initial formal notice to Daimler AG in April 2019, ordering the manufacturer to provide technical documentation for all certification tests conducted on the OM651 and OM626 engine families between 2013 and 2018. ^[6] Daimler's initial response characterised the toothpick findings as isolated quality-control anomalies and disputed that any systematic or sanctioned manipulation had occurred. This position became increasingly difficult to sustain as prosecutors from the Stuttgart Public Prosecutor's Office, who had opened parallel criminal proceedings in June 2019, obtained and disclosed internal engineering communications that referenced "HVE-12" in the context of pre-certification vehicle preparation procedures. ^[7]

In November 2020, the KBA issued a mandatory recall order covering approximately 260,000 vehicles in Germany, requiring an over-the-air software update to EGR control parameters and, for vehicles whose EGR hardware showed signs of abnormal wear potentially attributable to the alleged manipulation, a dealer-level hardware inspection. Daimler complied with the recall while continuing to contest the characterisation of the practices involved as wilful regulatory evasion. ^[5] The €870 million administrative fine, imposed in August 2021 and accepted by Mercedes-Benz Group AG without admission of criminal liability, was structured to include €210 million in disgorged profits attributed to vehicles sold with non-compliant emissions profiles and a €660 million penalty component. ^[1]

Criminal proceedings against fourteen named engineers and three senior managers remained ongoing as of the end of 2023, with trial dates in Stuttgart set for 2025. Separately, a class-action claim filed in the United Kingdom on behalf of approximately 130,000 affected vehicle owners was certified for trial by the Competition Appeal Tribunal in 2022, drawing comparisons in legal commentary to the UK diesel emissions group litigation arising from Volkswagen's Dieselgate settlement. ^[9]