Firstly it's useful to have an idea of nature of the problem with diesel car emissions. The real world emissions of different new diesel cars and vans vary wildly.
Lets have a look at chart below from the ICCT in September 2017. It shows the real world emissions of new cars that should conform to the Euro 6 standard NOx emissions standard of 80mg.
The chart shows that at the 2 extremes around 10% of Euro 6 cars actually comply with the legal Euro 6 limits when they are driven, while the worst cars are 24x over the limit. Most cars are in between and all are referred to by the car industry as "clean Euro 6 diesels"