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Air pollution and car emissions

Cars that produce the most CO

By Adrian Porter

Article 3 of 3

Petrol cars produce a lot of carbon monoxide (CO). In our tests, which are tougher than the official tests, one in five models produced CO in quantities that exceeds all emission limits from the past 27 years. 

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Air pollution from diesel cars might be making the headlines, but that doesn't mean that petrol cars are innocent.

While petrol cars produce very little in the way of NOx (apart from a very small number, see our round-up of the cars that produce the most NOx), petrol cars do produce carbon monoxide (CO) in large quantities.

Our carbon monoxide emissions tests have revealed:

  • In our tests, more than 20% of petrol cars we've assessed since 2012 produce more CO than would be officially allowed in 1993.
  • This includes four petrol-hybrid cars.

Keep reading to find out more about our tests and to discover which cars produced the most CO emissions in them.

If we find a car with very high CO emissions, we won't make them Best Buys. To find your next car that doesn't have massive amounts of emissions, head for our best cars.

Cars and CO emissions explained

Carbon monoxide (CO) is a colourless, odourless and highly toxic gas. The risk of suffering ill effects from the amount of CO produced by cars is very low (unless the car is running in an enclosed space, such as a garage), but CO itself can be fatal. CO, just like NOx, is one of the gasses limited by emission laws.

Euro emissions laws limit the amount of CO cars can produce (in official test conditions). This started with CO being limited to 2.72g/km in 1993 when Euro 1 was introduced. Years later, when Euro 2 replaced Euro 1, the limit was dropped to 2.2g/km. We are currently on Euro 6 and the limit is 1g/km of CO. 

Our tough tests are more challenging than the official tests and include a unique motorway cycle. We believe them to be more realistic and therefore more indicative of what your car actually produces. 

In our tests, two thirds of petrol cars produced more CO than the Euro 4/5/6 limit (the CO limit has not changed since Euro 4). Even more surprisingly, 22% emit more CO emissions than the Euro 1 limits from 1993.

However, this doesn't mean the cars are breaking the law, as the cars only have to beat the emission limits under the official tests.

Carbon dioxide (CO2) is a different gas that is much more harmful to the environment than to people or animals. There’s been some confusion in the media between CO2 and the other emissions. 

But it’s best to think of CO2, which is primarily bad for the environment, as separate from CO and other emissions (including NOx). CO and other emissions are particularly bad for human health.

Latest tests: the petrol cars that produce the most CO

Since updating our test programme for 2017, these are the most and least polluting petrol cars we've tested:

The car that produced the most CO in our tough tests

Despite being (relatively) small, the Ford Ka+ is the petrol car that creates the most CO in our tests.

When we questioned Ford, a spokesperson said: 'Thanks to massive Ford investment, each generation of vehicle is more advanced than the last. All new Ford cars coming to market meet the world’s strictest and most accurate emissions measurement regime.'

We will keep this page updated with the cars that produce the most and least amount of CO from our latest test programme.

We updated how we test cars from the start of 2017, making them tougher. The results are not directly comparable with the figures from our older tests. For more information, see how we test mpg and emissions.

Tested between 2012-2016: the 10 petrol cars that produced the most CO

The Nissan Note and Toyota Aygo may be small, low-powered cars – but in our tests they produced more CO than a Porsche 911.

Which? emission and MPG (fuel efficiency) tests aim to be more realistic than the official ones. We aim to test cars as consumers would use them, making use of tougher test cycles, which include a unique motorway segment. For more information on our tests and how they differ from the official European tests, go to how we test mpg and emissions.

All of the cars listed below exceeded the Euro 5 and Euro 6 limit in our tests. The Euro 5 and Euro 6 Limit is 1g/km.

The Euro 5 and Euro 6 Limit is 1 g/km.

(The CO emissions listed refer to the results in our stringent tests. These differ from the official tests, but we believe they are more realistic.)

Subaru Levorg, 1.6-litre (2015-): emits 7.2 g/km of CO
A car with a Euro 6 engine should be among the cleanest you can buy. But, unbelievably, the Subaru Levorg tops our CO emissions list thanks to this car's ability to produce more than seven times the limit of CO in our tests.

Hyundai Veloster, 1.6-litre (2012-2014): emits 6.11g/km of CO
The Veloster produces six times as much CO as the limit.

Nissan Note, 1.2-litre (2013-): emits 5.36g/km of CO
The 1.2 engine may sound small, but it’s big enough to produce more than 5.3g/km of CO – more than five times the limit.

Toyota Aygo, 1.0-litre (2014-): emits 4.91g/km of CO
This city car pushes out a massive amount of CO.

Alfa Romeo Giulietta, 1.4-litre (2010-): emits 4.69g/km of CO
Romeo, Romeo, quite a gassy Romeo. Produces four-and-a-half times the legal limit of CO.

Nissan 370Z, 3.7-litre (2009-): emits 4.63g/km of CO
This sporty number creates nearly as much CO as the Giulietta, above.

Hyundai i20, 1.2-litre (2009-2014): emits 4.61g/km of CO
Another small car, it too creates four-and-a-half times as much CO as it should.

BMW 4 Series, 2.0-litre (2014-): emits 4.53g/km of CO
The 242bhp engine of this BMW also creates four-and-a-half times the limit of CO.

BMW 2 Series convertible, 2.0-litre (2014-): emits 4.33g/km of CO
The 2 Series generates a near-identical amount of CO to the larger 4 Series.

Porsche 911 Carrera, 3.8-litre (2011-): emits 4.2g/km of CO
The 911 produces four times the limit of CO.

Hyundai i10, 1.0-litre (2014-): emits 4.09g/km of CO
A 1.0-litre city car it may be, but it creates just as much CO as the Porsche.

Lexus LS, petrol-electric hybrid (2013-): emits 3.95g/km of CO
Despite this Lexus being a petrol-hybrid, it generates four times the limit of CO.

Best Buys removed

Any car that that exceeds the 2001 Euro 3 limits in Which? tests can't be a Best Buy. Our tests are more realistic and reflect how drivers use their cars and differ from the official Euro standard tests.

We don't think a petrol car that can't keep its CO emissions below the earliest emission regulations from this century in our tests is worthy of being a Which? Best Buy.

Want a petrol car but without the excessive CO? Find your next car among our Best Buy cars.