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Which? tests reveal the worst diesel cars for air pollution

Which? NOx emission tests reveal Renault and Jeep are among the worst offenders of the carmakers tested

Which? tests reveal the worst diesel cars for air pollution

Despite officially meeting official emission limits, NOx emissions from the 16 Renault diesel cars we tested are seven times higher than the Minis tested.

There are emission laws in place across Europe to limit the amount of NOx (includes NO2) emissions produced by cars. Despite the same limit applying to all cars regardless of make, size or shape, Which? tests have revealed huge differences in the amount of NOx emissions produced by diesel cars from different brands.

Unlike the existing official car emissions test, Which? emission tests replicate the way drivers really drive their cars.

We’ve found that while the latest diesel cars are, on average, much cleaner than older models, car brands such as Renault, Jeep, Dacia, Nissan and Land Rover emit far more emissions than other brands in our tests.

Which? is taking a stance against high polluting cars. Any individual vehicle that produces more emissions than the 2001 Euro 3 limit in our tests cannot be a Best Buy. Find a car you can love by heading for our best cars.

Diesel cars that produce the most NOx

In the graphics below, you’ll find brand averages for the diesel cars we tested between 2012 and 2016. These are divided into cars that either officially meet the older and slightly more lax Euro 5 emission limits or the newer, more stringent Euro 6 emission limits.

Euro 5 diesel NOx limit: 0.18 g/k

Euro 6 diesel NOx limit: 0.08 g/km

Cars that meet the Euro 6 emission limits should be the cleanest cars you can buy. But as you can see from our test results, the NOx emissions from the car brands vary enormously. Scroll down to read our analysis of each brand.

Key car emission findings

Here is our analysis of what we found when we tested each brand.

Renault and Dacia

  • For brands on which we’ve collected data for both Euro 5 and Euro 6 diesel cars, Renault is the biggest polluter in our tests.
  • From the 16 diesel Renault cars we’ve tested, there is very little difference between the averages of its Euro 5 and Euro 6 cars.
  • In our tests, Renault’s newer Euro 6 diesels are, on average, dirtier than the averages of Euro 5 diesel cars from other brands.
  • The two Euro 6 Dacia cars we tested proved to have comparably high NOx emissions to Renault. Which may not be a coincidence, as Dacia is owned by Renault.


  • We’ve tested six Nissans with Euro 5 diesel engines and found them to have the second-highest NOx average across the board.
  • Renault also has a strategic partnership with Nissan, the Renault-Nissan Alliance, and the two brands co-develop engines.


  • Jeep almost broke the scale. The two Euro 5 cars we’ve tested manage to emit, on average, nine-and-a-half times the Euro 5 limit they met in official tests.

BMW, Mini and Toyota

  • The 33 BMW and BMW-owned Mini cars we’ve tested have some of the lowest NOx averages for diesel cars.
  • The BMW 3 Series was the ninth most-sold diesel car of 2016,  according to data from the Society of Motor Manufacturers and Traders (SMMT).
  • Toyota diesel cars also have low NOx emissions.

VW emissions

  • Mainstream VW Group cars (VW, Audi, Seat and Skoda) have some of the lowest NOx averages in our tests. But having been proved to have a defeat device in their Euro 5 cars, a question mark remains over these results.


  • We recorded a (comparatively) low NOx average across the seven Euro 5 Volvo cars we’ve tested. But the four Euro 6 Volvo cars we tested actually emit more NOx on average than its Euro 5 vehicles.


  • Our tests show NOx emissions from Ford diesel cars are quite high, based on the 12 models we’ve tested.
  • Ford sells two of the ten most-sold diesel cars in the UK; the Focus and the Kuga.

For more information about our findings, head for our guide to the dirtiest and cleanest carmakers.

Why air pollution matters

Air pollution is a very real problem in the UK; it only took five days for Brixton Road in London to break its NO2 (nitrogen dioxide) limit for the entirety of 2017. Estimates of fatalities from air pollution in the UK range up to 40,000 per year.

Which? versus official tests

Our emission tests are more realistic than the official ones. This is because current tests are based on the outdated and soon to be replaced NEDC cycle, which doesn’t really strain the vehicle being tested. The official rules are also lax and allow for numerous loopholes to be exploited by manufacturers, such as running the car in eco-mode, turning off air-con and over-inflating tyres.

In comparison, Which? tests use much more realistic cycles, including the more strenuous WLTP cycle and a motorway cycle where the car is accelerated to and then sustains motorway speeds.

No link between MPG and emissions

It also might surprise you to learn that our unique testing has revealed there is no strong link between the level of emissions your car creates and its MPG (fuel economy). We’ve even found hybrids that produce high levels of toxic emissions.

Want to know which cars have the best fuel economy? Head to our most fuel efficient cars.

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