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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 exceed 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.

  • In our emissions 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.

If we find a car with very high CO emissions, we won't make it a Best Buy. To find your next car that doesn't produce high emissions, head for our best cars.

Car emissions and CO 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 car-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 our tests 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% emitted more CO emissions than would be allowed under the Euro 1 limits from 1993.

However, this doesn't mean the manufacturers of those 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 car emission 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

The 2.5-litre, 175hp petrol engine we've tested in the Subaru Outback (2015-) officially meets Euro 6d-temp, the latest emission standard that all cars will have to meet as of September 2019.

That might make you think the petrol engine in this Subaru is clean, but nothing could be further from the truth: it's the car with the highest CO emissions under our current test programme.

Our assessments differ from the official ones and include a dedicated motorway test, and that is exactly where we caught out this Subaru.

Producing 7.6g/km of CO in our tests, it's broken the record for the most CO ever recorded from a vehicle since we started recording emissions back in 2012.

To put it in perspective, the Toyota Prius hybrid (2017-), the petrol car with the lowest CO level we've tested, produces over 300 times less CO than this Subaru.

It's also worth noting that the most CO-producing petrol car in our previous test programme (2012-2016) is also a Subaru (scroll down for more details).

In fact, the car that produces the most NOx in our current test programme is a diesel-powered Subaru, as is the car producing the second-highest amount under our previous test programme.

It seems Subaru needs to work on its emissions-reducing technology.

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 the way we test cars from the start of 2017, making the tests tougher. This means 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.

Many of the engines tested under this test programme will either be unavailable as new, or be phased out by September 2019. However, they will still be available to buy on the used-car market.

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 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 1g/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. The Giulietta produces more than 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 similar 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 almost 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 almost 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 than the official Euro standard tests, and reflect how drivers use their cars.

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.