Technical Deep Dive: The Specific 'Defeat Devices' Alleged in the High Court Case Against Major Carmakers

A pivotal trial has commenced at the London High Court against some of the world's largest car manufacturers, marking a significant new chapter in the decade old "Dieselgate" emissions scandal. This mass legal action is being brought on behalf of approximately 1.6 million vehicle owners in the UK who allege that their diesel vehicles were fitted with illegal "defeat devices." These devices, lawyers claim, allowed the cars to pass official nitrogen oxide (NOx) emissions tests while polluting far more heavily during real world driving. The trial focuses on five lead defendant carmakers, with the ruling expected to set a major precedent for similar claims against a total of fourteen manufacturers.

The lead defendants in this landmark three month trial are Mercedes Benz, Ford, Nissan, Renault, and the Stellantis-owned brands Peugeot and Citroen. The claimants’ legal teams assert that these companies used prohibited defeat devices, which detected when the vehicle was under laboratory testing conditions and adjusted the engine’s emissions controls accordingly. Once on the road, the systems were allegedly disabled or significantly reduced, causing the vehicles to emit NOx levels far exceeding legal limits. All five lead manufacturers vigorously deny the allegations, arguing that their emissions control systems were legally and technically justified, and necessary to protect the engines from damage. This massive legal action is one of the largest group claims in English legal history, with the total value of the litigation against all manufacturers previously estimated to be worth at least £6 billion.

The consequences of this case, if the claims are proven, are far reaching. For the automotive industry, a finding of liability would confirm that the practice of emissions cheating, first exposed with Volkswagen in 2015, was a more widespread industry issue. The original Volkswagen scandal cost that company over £32 billion in global fines, legal costs, and compensation. A ruling against the current defendants could pave the way for hundreds of thousands of additional compensation payouts for motorists who bought or leased diesel vehicles based on false environmental claims. For the public and the environment, a key issue is the real world health impact of the higher than reported nitrogen oxide emissions, a known contributor to air pollution and respiratory illnesses, particularly in urban areas.

The trial's current phase will focus on determining the central question of whether the manufacturers used unlawful "defeat devices." This liability ruling is expected in the summer of 2026, with a separate trial on the quantum (level of compensation) to follow later that year. Regardless of the outcome, the case highlights the ongoing tension between meeting increasingly stringent environmental regulations and the commercial pressures within the global car manufacturing sector. It underscores the critical need for robust, real world emissions testing and transparency to maintain consumer trust and ensure public health. The industry continues to face immense pressure to accelerate the transition to genuinely clean, zero emission vehicles.