Press release

“Suspicious” emission levels were found in at least 77% of tests of diesel cars in Europe

The “suspicious” levels of nitrogen oxide emissions indicate the “likely use” of what is now considered a prohibited defeat device, according to the latest Dieselgate rulings of the EU’s highest court.   

Berlin, 22 March 2023— “Suspicious” nitrogen oxide (NOx) emission levels were found in at least 77% of tests performed on diesel cars in Europe. These excess emissions indicate the “likely use” of an engine calibration strategy that may now be classified as a prohibited defeat device based on recent rulings of the Court of Justice of the European Union (CJEU).

A new study, carried out by the non-profit research organization International Council on Clean Transportation (ICCT) and published today, assesses the magnitude of “suspicious” and “extreme” emission levels shown in post-Dieselgate testing performed by government and independent organizations since 2016. The emission levels are defined based on expected emission behavior for given types of tests.

Approximately 53 million diesel cars were sold in Europe from 2009 to 2019, emitting high levels of NOx, a hazardous air pollutant that poses a significant risk to human health. A companion analysis by the ICCT estimates that 19 million vehicles with “suspicious” emissions remain in use today – of these, 13 million vehicles were models showing “extreme” emissions. The issue of excess NOx emissions is prevalent across manufacturers.

According to the investigation, not only did a large majority of tests (85% for Euro 5 and 77% for Euro 6 diesel cars) show “suspicious” excess emissions, but also many showed “extreme” emission levels. “Extreme” emission levels were found in at least 40% of official tests of diesel cars, indicating the “almost certain” presence of a calibration strategy that may now be considered a prohibited defeat device.

The ICCT also assessed data collected through past remote sensing campaigns, which measure the real-world emissions of vehicles from the roadside as drivers pass. The data, consisting of 700,000 measurements in five European countries, showed that approximately 75% of diesel car engine families’ emission averages were found to exceed the “extreme” threshold.

It is difficult to dispute what is a large amount of analyzed data and testing collected from multiple sources. All of them point in the same direction. After the CJEU rulings, these results present a solid body of evidence for authorities to investigate and potentially take corrective action to address health risks posed by European diesel cars driven on our roads”, says Peter Mock, ICCT’s Europe Managing Director.

A defeat device is a software code installed in the vehicle to alter or deactivate the emissions control system under certain operating conditions. The use of defeat devices was at the center of the 2015 Dieselgate scandal.

European courts continue to review evidence from related cases. In a ruling released in December 2020, the CJEU clarified the conditions under which defeat devices are prohibited, even for vehicles sold prior to the decision. It stated that “only immediate risks of damage which create a specific hazard when the vehicle is driven… justify the use of a defeat device” (C-693/18). The judges broadly limited the scope of exemptions. Additional CJEU rulings in July 2022 further clarified that defeat devices cannot be justified in any event if they operate throughout most of the year during normal driving conditions.

“The levels of emissions found in the tests offer wide-ranging data to estimate the prevalence of prohibited defeat devices. What we found is that vehicles surpassing the “extreme” threshold is a red flag and should pose questions about the rationale for their emission behavior. A total of 150 vehicle models show emissions above it and represent about 13 million vehicles still on the road in the EU-27 and UK,” stated Yoann Bernard, Senior Researcher at the ICCT.

The ICCT report also analyzed the statements presented by manufacturers during investigations conducted by national market surveillance in four European countries. From these statements, the ICCT identified 66 vehicle models that used calibration strategies that are now considered prohibited defeat devices based on the latest CJEU rulings because they function during normal driving conditions. Most of these vehicle models (48) use emission calibration strategies that alter or deactivate the emissions control system in low ambient temperatures which are typical within the EU.

EU law mandates that Member States have the responsibility to investigate cases of potential defeat devices and to require manufacturers to take corrective action if a prohibited defeat device is found. The 2020 ruling from the CJEU still applies in the UK as retained EU law, and therefore must inform the UK government’s interpretation of rules prohibiting the use of defeat devices.

Since the Dieselgate scandal, only some manufacturers have performed recalls and updates of diesel cars in Europe. The impact of these updates is unclear as many vehicles have not been retested, or the testing results have not been released publicly.

Air pollution continues to be the biggest environmental health hazard in Europe. Diesel vehicles are a major contributor, largely due to high NOx emissions. In the region, 35,400 premature deaths were linked to on-road diesel vehicle emissions in 2015 or 14% of all air pollution-related premature deaths.


Note to the editors:

The “extreme” threshold indicates a level of emissions so far in excess of regulatory limits that explanations for it other than the presence of a defeat device are highly unlikely. Circumstances that could lead to extreme emissions without a defeat device could exist, such as undetected individual vehicle malfunctions, undetected aftertreatment system regeneration during the test, measurement instrument malfunctions, or a widespread malfunction across a vehicle family. Such conditions are exceedingly rare, but always theoretically possible. To acknowledge the remaining small degree of uncertainty, we, therefore, conclude that tests and vehicle models crossing the extreme threshold indicate that the use of a defeat device is almost, but not perfectly, certain.

Media contact
Susana Irles,

Publication details
Reassessment of excess NOx from diesel cars in Europe following the Court Justice of the European Union rulingsAuthors: Michelle Meyer, Yoann Bernard, John German, Tim Dallmann

Available in English, German, French, Spanish, and Italian.

Blog post: It’s time for Europe to address diesel defeat devices once and for all

Please use this link when citing the report.

About the International Council on Clean Transportation

The International Council on Clean Transportation (ICCT) is an independent research organization providing first-rate, unbiased research and technical and scientific analysis to environmental regulators. Our mission is to improve the environmental performance and energy efficiency of road, marine, and air transportation, in order to benefit public health and mitigate climate change. Founded in 2001, we are a nonprofit organization working under grants and contracts from private foundations and public institutions.

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The ICCT thanks ClientEarth for providing legal expertise that helped to shape the scope of the report’s analysis and for valuable discussions regarding Court of Justice of the European Union rulings on defeat devices. For more details on ClientEarth’s work in this area, please visit:


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