The Dieselgate UK High Court has rejected most claims made by about 1.6 million car owners in one of the biggest legal cases in British history. The ruling marks a major step in the long-running emissions dispute that began after the global diesel emissions scandal first came to light more than a decade ago. The case involved claims against several major car makers. Owners argued that the companies used software or engine controls that lowered nitrogen oxide emissions during official tests but worked differently in normal driving. They said this allowed vehicles to meet legal standards during testing while producing higher emissions on the road. The legal action covered vehicles sold from 2009 by Mercedes, Ford, Renault, Nissan, and Peugeot-Citroën. The trial lasted 15 weeks between October 2025 and March 2026. It focused on 20 vehicles, but the judgment also applies to similar claims involving other manufacturers. Lady Justice Cockerill ruled that most of the engine strategies examined during the trial did not meet the legal definition of a prohibited defeat device. She said not every emissions control system or engine calibration should be treated as illegal under British law. The judge explained that claimants needed to show that manufacturers intended to make vehicles behave differently during official emissions testing. She said it was not enough to prove that an emissions control system worked less effectively outside test conditions. The court also noted that measuring the exact effect of different engine settings on emissions was difficult and that the available testing methods had limits. While most claims failed, the court found that some technology used in certain Mercedes and Peugeot-Citroën vehicles could qualify as prohibited defeat devices. That part of the ruling leaves open the possibility of further legal action involving those specific vehicles. Mercedes-Benz said the company welcomed the decision because the court ruled largely in its favor. However, it said it is reviewing the judgment after one of its vehicle systems was found to be non-compliant. The company also noted that the disputed function had already been removed from vehicles in 2015. Stellantis, the parent company of Peugeot-Citroën, said it is also considering an appeal over the findings related to its vehicles. Nissan said it has always maintained that the technology used in its diesel models did not amount to illegal defeat devices. Renault said its vehicles were designed and built to meet all legal requirements in force at the time. The legal teams representing the vehicle owners described the judgment as disappointing. They argued that the court adopted a narrower legal interpretation than courts in many European Union countries. Lawyers said this creates a different legal standard in Great Britain compared with much of Europe. The claim was brought by more than 20 law firms acting for affected vehicle owners. Lawyers said they are reviewing the judgment and considering whether to seek permission to appeal. They believe higher courts should examine how British law defines prohibited defeat devices. Campaign groups also reacted to the ruling. Environmental campaigners said the decision does not change the scientific evidence about harmful diesel emissions. They argued that nitrogen oxide pollution remains a serious public health concern and called on manufacturers to address affected vehicles where the court found legal violations. The Dieselgate scandal first gained worldwide attention in 2015 after investigators found that Volkswagen had installed software designed to recognize emissions tests and temporarily reduce pollution levels. The discovery led to billions of dollars in fines, recalls, compensation payments, and legal claims around the world. Since then, owners have filed similar lawsuits against several other manufacturers over the use of emissions control technology. The latest High Court ruling is one of the most important legal decisions in the UK Dieselgate litigation. While most manufacturers secured a favorable outcome, the judgment leaves some claims alive and could lead to further appeals. The final legal outcome may depend on whether higher courts decide to review the interpretation of emissions rules and defeat devices under British law.