Carmakers have broadly won the first round in UK diesel emissions litigation, as the High Court rejected most of the “dieselgate” claims against Mercedes-Benz, Ford Motor, Nissan Motor, Renault and Stellantis’s Peugeot-Citroën-DS division.
The ruling from the King’s Bench Division was issued by Lady Justice Cockerill after 55 hearing days held between October 2025 and March 2026.
It relates to claims brought by about 1.6 million owners of Euro 5 and Euro 6 diesel vehicles, broadly from 2012 to 2018.
Claimants said the manufacturers installed prohibited “defeat devices” intended to cheat emissions tests. Volkswagen is not a defendant in this trial.
The judge adopted a narrow interpretation of a defeat device, holding that it is “a device which senses one or more parameters of the test… and objectively operates with the purpose of causing the ECS to work more effectively when it senses that it is being subjected to a test cycle compared to how it works in out-of-test driving”.
Using that approach, the judgment concluded that most of the around 40 devices alleged across the five manufacturers were either not shown to be defeat devices, or that claimants did not prove a “reduction in effectiveness” as required under EU emissions regulations.
Outcomes varied by carmaker.
In the cases against Mercedes and Ford, none of the alleged devices were upheld.
For Peugeot-Citroën-DS, a “split mode” device was found to be a prohibited defeat device.
For Renault and Nissan, several devices, including torque cut-off systems and correction factors, were found to breach the regulations, with reduction in effectiveness established in those cases.
The proceedings form part of a wider group litigation structure covering diesel emissions claims against a broader set of manufacturers.
Mercedes, Ford, Peugeot-Citroën and Nissan-Renault were selected as lead defendants to help guide the remaining cases.
Reuters reported that Mercedes-Benz welcomed the decision, with a spokesperson saying the court had found "very largely in favour" of the company.
Mercedes added that it disagreed with the adverse finding made against it and was weighing its next steps, including a possible appeal.
Stellantis said just one allegation had been upheld against it and that it was considering whether to seek permission to appeal.
"Stellantis maintains its firm position that all its vehicles comply with applicable emissions regulations," the company was quoted as saying.
Reuters also reported that the claimants’ lawyers were assessing whether to appeal Friday’s decision, arguing that it "adopted a significantly narrower interpretation of the law than that applied elsewhere in Europe".
The hearing, which opened in October, was the biggest so far across 13 groups of claims.
Although the latest judgment dealt with test cases against five manufacturers, it will also influence about 800,000 related claims against other carmakers.
A further trial is scheduled for October to address the consequences of any emissions rules breaches and whether claimants are entitled to damages.
Volkswagen’s “Dieselgate” scandal, which has cost the German carmaker billions of euros, emerged in 2015 and triggered years of litigation.
Other manufacturers have argued that the cases against them differ fundamentally from Volkswagen’s emissions-cheating scandal, and that emissions control systems can legitimately function differently in certain conditions.
