High Court rejects diesel emissions allegations against carmakers

Judge rules that specific technology used by Mercedes-Benz and Peugeot-Citroën amounted to prohibited defeat devices, but rejects most of the other allegations examined at trial. 

The High Court has rejected most of the allegations examined in a major diesel emissions case involving claims brought by around 1.6 million UK drivers. 

The judgment found that most of the emissions-control technology considered at trial did not amount to a prohibited defeat device.  However, the court ruled against Mercedes-Benz and Peugeot-Citroën over specific technology used in some of their vehicles.

Drivers allege that their diesel cars were fitted with prohibited defeat devices that reduced the effectiveness of emissions-control systems outside regulatory testing conditions, leading to higher nitrogen oxide emissions during normal use. The manufacturers deny liability and say their vehicles complied with relevant rules.

Martyn Day of Leigh Day, one of the firms representing drivers, said:

“’The high court has found that despite their denials, other manufacturers also used VW-style defeat devices. The judge has also found that many other defeat devices are lawful because she surprisingly disagreed with the established EU case law on what a defeat device is.”

Lady Justice Cockerill said the court had to decide when software that changes the operation of an emissions-control system amounts to a defeat device.

She concluded that not every calibration or emissions-control strategy falls within that definition. The software must be designed to recognise when the car is undergoing an emissions test and make the emissions-control system work more effectively during testing than during normal driving. 

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What the court decided on emissions test defeat devices

The trial involved lead claims brought against manufacturers including Mercedes-Benz, Ford, Peugeot-Citroën, Renault and Nissan. The court examined a number of sample Euro 5 and Euro 6 vehicles manufactured between 2012 and 2017. It was the first substantive liability trial in the wider Pan-NOx group litigation, which includes cases against a range of other vehicle manufacturers.

For most of the sample vehicles, the court found the challenged technology did not amount to a prohibited defeat device. 

However, it the court found that two specific types of technology did break the rules:

• a coolant temperature setting used in some Mercedes-Benz vehicles, which was removed through a software update in December 2015; and
• a “split-injection” strategy used in some Peugeot-Citrë Euro 5 vehicles.

The judge also considered what the outcome would have been under a different interpretation of the law. On that approach, a larger number of defeat devices would have been established, including devices in vehicles made by each of the lead manufacturers.

However, the court said many allegations would still have failed for other reasons. These included a lack of evidence that the technology reduced the effectiveness of emissions controls under normal driving conditions.

Lawyers representing drivers argue that this shows the result turned heavily on the judge’s interpretation of the legal definition of a defeat device. 

What this means for people with a diesel emissions claim

This ruling does not decide the outcome of each driver’s claim or how much compensation anyone may receive.

It dealt with preliminary legal questions, including the meaning of a defeat device, whether particular emissions strategies fell within that definition and whether breaches of the legislation could allow individual drivers to bring claims.

Importantly, the court found that the legislation is capable of giving drivers a private right of action. However, it said further evidence would be needed before it could reach conclusions about contractual claims.

Anna Varga of Pogust Goodhead, another firm representing drivers, said the judgment “does not bring this litigation to an end.” 

She said the findings against Mercedes-Benz and Peugeot-Citroën remained significant, despite the court adopting a narrower interpretation of the law than courts elsewhere in Europe.

People with claims against Mercedes-Benz, Peugeot-Citroën, Ford, Renault or Nissan should receive further information from their legal representative once the judgment has been fully reviewed. 

The ruling may also affect claims involving other manufacturers within the wider litigation, although the consequences for those cases are not yet clear.

What happens next in the diesel emissions case

The judgment was issued subject to editorial corrections, with a final version to follow.

A further trial is currently scheduled for October 2026. It is expected to consider the consequences of any actionable breaches, including questions relating to compensation and other remedies.

Lawyers on both sides may appeal the decision, which could affect the timetable.

People who have an active claim do not need to take any immediate action. Given the number of drivers involved, it is likely to be best to wait for an update directly from the law firm handling the claim.

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