Lady Justice Cockerill has ruled that most of the diesel emissions cheating allegations brought by 1.6 million UK car owners against a host of major manufacturers are unfounded – but found that some Mercedes and Peugeot-Citroën vehicles did contain prohibited defeat devices.
In a 369-page judgment handed down today, the judge said “in the majority of instances, the court found that the relevant strategy did not constitute a prohibited defeat device”. The ruling came after a ten-week trial that concluded in March, focusing on 20 sample vehicles from five manufacturers: Mercedes-Benz, Renault, Nissan, Ford, and Peugeot and Citroën.
“Judge rejects most dieselgate claims against major carmakers but finds defeat devices in some Mercedes and Peugeot-Citroën vehicles.”
The case – the largest group action trial in English legal history, heard over 15 weeks between October 2025 and March 2026 – was brought by motorists who bought, leased or otherwise acquired diesel vehicles from 2009 onwards. The 880,000 claimants argued that the cars contained software designed to detect when they were being tested and reduce harmful nitrogen oxide (NOx) emissions, only to revert to higher emissions on the road. The manufacturers denied using prohibited defeat devices.
However, the court found that proving intention to cheat was essential. “For a defeat device to be found, there needs to be an intention to cause the emissions control system to operate differently when it senses it is being tested,” Cockerill said. “It was not enough for the Claimants simply to establish that the challenged strategies reduced the effectiveness of emissions-control systems outside the relevant testing conditions.”
The judge noted that “if an alternative approach to the meaning of ‘defeat device’ were taken, a larger number of devices would be established, including devices in each of the lead manufacturers cars.” But she concluded that, bar two exceptions, the strategies used did not meet the legal definition. The exceptions were a strategy in Mercedes cars that was removed in 2015, and another used in some Peugeot-Citroën vehicles.
Mercedes-Benz welcomed the ruling but said it disagreed with the finding on one of its four sample vehicles. “In our view, the emission control software functionalities are justifiable on both technical and legal grounds. We are actively considering all of our available options, including a potential appeal,” the German carmaker said. Stellantis, owner of Peugeot-Citroën, also said it was considering an appeal over the upheld allegations. Nissan said it had always maintained that its vehicles did not contain defeat devices.
James Oldnall, managing partner at Milberg, which represented some of the claimants, said: “We are pleased that the court has ruled that Mercedes installed illegal defeat devices, just like Volkswagen.” The claimants’ lawyers are considering appealing, arguing that the judgment creates “a significant divergence” between the legal position in Great Britain and much of Europe.