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Cars & travel.

Low-emission cars checker

Want to buy a low-emission car? Our tests show that death-causing NOx emission levels are plummeting from the latest cars, but planet-harming CO2 from many of the same cars is actually on the rise. Our free low-emissions checker will help you find the genuinely low-emission cars on the UK market.
Adrian Porter
Car emissions explained 1

What is a low-emission car? Before the 2015 VW emissions scandal, also known as dieselgate, a low-emission car simply produced a small amount of CO2 (carbon dioxide).

But now we're more aware of pollutants like NOx (oxides of nitrogen), PM (particulate matter) and CO (carbon monoxide) and how poor air quality contributes to tens of thousands of premature deaths in the UK.

The good news is that our tests have found that, on average, the cars that meet the latest emission regulations (Euro 6d-temp and Euro 6d, explained in more detail below), are producing a fraction of the NOx and CO of the cars they replaced.

But to make things more complicated, our tests show that CO2 emissions from these very same cars are actually going up, not down.

In a nutshell, we may have saved humanity's lungs at the expense of our planet's health. It also makes buying a low-emission car a lot more complicated.

It's not all doom and gloom though. Not only do we expect cars to become more fuel efficient over the next few years as technology is refined, there are some cars available today that strike a balance of low CO2 and low air pollutants such as NOx and CO. Our free low-emissions car checker, below, reveals CO, NOX and CO2 for hundreds of cars we've tested, so you can make an informed decision on your next purchase.

Want a car that produces zero, rather than low, emissions? You'll need to go fully electric. Take a look at our pick of the best electric cars

NOX, CO and CO2 car emissions checker

Our free low-emissions checker shows all emissions that our independent tests have measured for every car we've tested since 2012. This tool shows:

  • the car model and all engines we've tested in that car
  • the Euro emission legislation the car meets in official tests
  • the amount of NOx and CO each engine produced in Which? independent tests since 2012
  • the amount of CO2 we measured from the tailpipe for all cars Which? has tested since 2017 – this will be updated in the future to include all cars back to 2012.

CO2 figures are given in g/km, while NOx and CO emissions are rated from 'very low (trace)' to 'extremely high'. Scroll down to below the tool to find out more about what we consider to be ‘high’ and the consequences for those cars in our tests.

Car emissions checker – check by make and model

Every car we review is driven by our experts and put through our independent lab tests. As our lab tests are objective, you can trust our results. See our expert pick of the best cars.

Using our low-emission checker: high and low car emissions explained

New cars must meet the current emission regulation in official tests, otherwise they cannot be sold.

Our tests are tougher than the official ones, but we use the official Euro limits as benchmarks against our own test results to give them context.

Any engine that has a rating of ‘high’ produced more pollutants in our tests than the Euro 3 limit from 2001. So we withhold Which? Best Buy car status.

Euro limits are explained in more detail below. But, in brief, official emission regulations limit the amount of emissions such as NOx and CO in official tests.

(We updated our tests in 2017. Figures from our 2012-2016 test programme aren't directly comparable with figures from the current test programme.)

High and low car emissions in Which? tests explained

 Which? car emission levels explainedNOxCOEquivalent to
Extremely low (trace)Less than 0.015g/kmLess than 0.015g/kmNOx emissions are at least five times less than the current Euro 6 diesel limit. Or nearly seven times less than than Euro 6 petrol CO limit.
Very lowLess than 0.08g/kmLess than 0.05g/kmProduces less NOx than the current Euro 6 diesel limit
LowLess than 0.18g/kmLess than 0.5g/kmProduces less NOx than the Euro 5 diesel limit
MediumLess than 0.5g/kmLess than 1g/kmProduces less NOx than the Euro 4 diesel limit, or less CO than the Euro 6 petrol limit (CO petrol limit unchanged since Euro 4).
HighLess than 0.97g/kmLess than 2.72g/kmProduces more NOx than the Euro 3 diesel limit, or more CO than the Euro 3 petrol limit.
Extremely highMore than 0.97g/kmMore than 2.72g/kmProduces more NOx than the Euro 1 diesel limit, or more CO than the Euro 1 petrol limit.

Table notes:
1 Euro 1-6 limits are used for comparative purposes only, to place our own independent test results in context.
2 Which? tests are tougher than the official ones. Go to our guide on how Which? tests mpg and emissions to find out more.

The five lowest-emission cars

It's still possible to buy a low-emission car without going fully electric.

Taking CO2, CO and NOx measured from our tests into account, it's the five cars below all have the lowest all-round emissions we've tested on our current test programme.

All have small or trace amounts of NOx and CO, and reassuringly low CO2. This means they are great for our lungs, better for our planet than most and will be kind to your fuel bills.

The Toyota Plug-In Hybrid has been at the top of the list since we first put it together. A spokesperson told us: 'Our full hybrid electric technology provides customers with excellent overall environmental performance at an affordable price – allowing them access to electrification in a convenient and practical way and have a positive impact on both CO2 and air quality.'

Toyota Prius Plug-In Hybrid (2017 to present)

  • Engine: 1.8 Plug-in hybrid (Euro 6b)
  • Measured emissions: NOx (0.004g/km), CO (0.025g/km), CO2 (58g/km)

When looking across all emissions we measure – CO, NOx and CO2 – the plug-in version of the Prius is the cleanest car we've tested to date. The only problem is that the model we tested is the Euro 6b version of the car and can't be bought new any more, so you'll need to look for it on the used car market.

Aside from the Hyundai Ioniq Plug-in Hybrid – which had worryingly high CO emissions in our tests, so can't feature in our top five despite its strong CO2 performance – this Prius has the lowest CO2 value we've measured from a car (with the exception of zero-emissions electric and hydrogen cars). This is also going to make it among the most affordable traditional-fuel cars to run.

We've not tested a newer Euro 6d engine in the plug-in hybrid as yet, but we have lab tested the Euro 6d-temp in the regular Prius. While it just falls short of appearing on this list, it is another remarkably clean engine – and so we have high hopes for the current engine in the Toyota plug-in hybrid.

Read our Toyota Prius Plug-In Hybrid review to find out what this car's like to drive and what fuel economy figures you can expect.

Mercedes-Benz GLE plug-in hybrid (2020 to present)

  • Engine: Plug-in diesel hybrid, 350de 4Matic 9G Tronic (Euro 6d)
  • Measure emissions: NOx (0.021g/km), CO (0.007g/km), CO2 (71g/km)

That's right, the second-cleanest car we've tested is a 2.7-tonne SUV with a 2.0-litre diesel engine under the bonnet (plus a 31.2kWh battery pack).

Starting with a full battery, we averaged a remarkable fuel economy in our independent tests. Unfortunately, and like every plug-in hybrid, that fuel economy will fall once the battery has depleted. In this case, it will fall drastically.

Although it has a diesel engine, its NOx emissions are very low, and CO is nearly non-existent. Coupled with the low CO2 output, this is a very clean SUV – provided you keep the battery topped up.  

Want a luxury SUV with genuine green credentials? Read our review of the Mercedes GLE diesel plug-in hybrid (2020-).

Mercedes-Benz B-Class plug-in hybrid (2020 to present)

Mercedes B-Class PHEV
  • Engine: Plug-in petrol hybrid, B250e AMG Line 8G-DCT (Euro 6d)
  • Measure emissions: NOx (0.006g/km), CO (0.028g/km), CO2 (85g/km)

The second Mercedes on the list isn't an SUV, but a people carrier (or MPV). For those unfamiliar with the B-Class, this car is based on the ever-popular A-class hatchback, but offers much more in terms of space and everyday practicality.

Combined with low fuel costs and emissions, it's a car that is both environmentally and family friendly. But don't fall into the 'well it must be boring' trap. It might be a conservatively styled MPV, but the 1.3-litre four-cylinder petrol engine combines with the electric motor to deliver 218hp of power and 450Nm of torque (pulling power). In short, it's nippy.

Could this be your new family car? Read our Mercedes-Benz B-Class Plug-in hybrid (2020-) to find out more.

Skoda Superb Estate plug-in hybrid (2020 to present)

Skoda Superb Estate PHEV
  • Engine: Plug-in petrol hybrid, 1.4 TSI iV DSG (Euro 6d)
  • Measure emissions: NOx (0.006g/km), CO (0.517g/km), CO2 (77g/km)

The first car from Skoda to feature any electrification is a marvel for how clean it is. Badged as the 'iV', its particularly low NOx and CO2 emissions see this large estate shoot straight into this list of the five cleanest cars we've tested to date.

It combines a 1.4-litre petrol engine with an electric motor, delivering a healthy (indeed quite potent for this type of car) power output of 218hp. It's also spacious and the boot is a decent size.

Read out Skoda Superb estate plug-in hybrid (2020-) review to find out if this low-emission load-lugger is the car for you.

Renault Mégane Sports Tourer plug-in hybrid (2020 to present)

Renault Megane Sports Tourer PHEV
  • Engine: Plug-in petrol hybrid, E-Tech Plug-in Hybrid 160 Auto (Euro 6d)
  • Measure emissions: NOx (0.001g/km), CO (0.341g/km), CO2 (87g/km)

Currently the only brand to have different models feature in both the five lowest-emitting cars and five highest-emitting cars (see below), Renault's Mégane Sports Tourer is a medium-sized estate that mixes a 1.6-litre petrol engine with two electric motors and a 9.8kWh battery to great effect.

Reasonably low CO2 and just a trace of NOx emissions make this a clean car. The battery pack isn't huge, so you'll need to top it up regularly to keep its impact on the environment to a minimum.

It's not perfect though. The space required for the battery pack does takes a 95-litre chunk out of the boot space compared with the non-hybrid version. While some boot space is often sacrificed to accommodate battery packs, many modern plug-in hybrids are able to mitigate that impact far better than this Renault does.

Find out more about this Renault Mégane Sports Tourer plug-in hybrid by reading our independent review

The five highest-emission cars

At the other end of the spectrum are the five highest-emitting cars we've tested on our current test programme. Although they've all been certified by the relevant EU emission regulation, they fared poorly in our own stringent tests.

In much the same way that Toyota makes the cleanest cars we've tested, it's Subaru that has made the three dirtiest we've seen. We approached Subaru for comment, but it wasn't comfortable commenting on the results as our tests differ from the official ones.

Subaru Forester (2013 to present)

  • Engine: 2.0-litre Lineartronic diesel (Euro 6b)
  • Emissions: NOx (2.022g/km), CO (0.022g/km), CO2 (189g/km)

While the 2.0-litre diesel boxer engine under the hood of the Forester has been discontinued, and the only engine type you can buy new is a hybrid, you'll still be able to find the diesel version on our roads and on the used car market.

In our tests, which are tougher than the official tests, this car produced an alarming amount of NOx – 25 times as much as the Euro 6 limit (which it met in official tests). Combine that with our tests revealing it has high CO2 emissions and you get the highest-emitting car we've tested since 2017. 

High-emitting cars like this one will never be recommended as a Best Buy. 

To find out more about this car, including its mpg in our stringent tests and what it's like to drive, see our Subaru Forester review.

Subaru Levorg Sport Tourer (2015 to present)

  • Engine: 2.0-litre Lineartronic petrol (Euro 6d-temp)
  • Emissions: NOx (0.008g/km), CO (8.417g/km), CO2 (180g/km)

The 2.0-litre petrol engine in the Subaru's Levorg Sport Tourer is proof that although a car meets stringent emission limits, that doesn't automatically mean it's clean, based on our tests.

Being a petrol engine, NOx is reassuringly low. But in our tests, the amount of CO it produces is off the charts, at nearly 8.5 times the Euro 6 limit (this figure is given for a comparison – in official tests, this car meets the Euro 6d-temp limit).

CO2 is also very high in our tests, which is why this car makes the number-two spot on our list.

Here's our expert, independent Subaru Levorg Sport Tourer review.

Subaru Outback (2015 to present)

  • Engine: 2.5-litre Lineartronic petrol (Euro 6d-temp)
  • Emissions: NOx (0.010g/km), CO (7.609g/km), CO2 (186g/km)

Another Subaru and another Euro 6d-temp car that, in theory, should be clean (and was in official tests). But it came up as one of the most polluting in our own, independent tests, which are tougher than the official ones.

The 2.5-litre petrol engine in the Outback is similar to 2.0-litre petrol engine in the Levorg in terms of the colossal amount of CO it outputs, along with a high figure for CO2.

Find out more about this rugged estate-come-SUV by reading our Subaru Outback review.

Ssangyong Rexton (2018 to present)

Ssangyong Rexton
  • Engine: 2.2-litre Diesel Sapphire 4WD Automatic (Euro 6b)
  • Emissions: NOx (0.617g/km), CO (0.007g/km), CO2 (250g/km)

In our tests, this car produced the highest amount of CO2 compared with any other car in this list, as well as a large amount of NOx.

This SUV may draw people in with its low purchase price, loads of space and reasonable interior, but its high emissions and other flaws may just put them off again.

Get the full picture by reading our Ssangyong Rexton (2018-) review.

Renault Grand Scénic (2016 to 2020)

Renault Grand Scenic (2016-2020)
  • Engine: 1.6-litre diesel, Energy DCI 160 Bose Edition (Euro 6b)
  • Emissions: NOx (0.896g/km), CO (0.026g/km), CO2 (160g/km)

The seven-seat version of the regular Scénic MPV is the fifth most-polluting car we've tested since 2017.

Just like the regular Scénic, the Grand Scénic has been discontinued, although you'll still find it on the used car market, and probably at an appealingly low price.

But if one of your buying consideration is low emissions, ignore the price tag, as we found this car's high NOx and CO2 emissions make it a highly polluting modern car.

Find our more by reading our Renault Grand Scénic (2016-2020) review.

Car emissions explained: how exhaust emissions affect us

Car emissions we measure in our tests can be divided into two main groups:

Air pollutants

NOx (oxides of nitrogen)

NOx is comprised of two gases: nitric oxide (NO) and nitrogen dioxide (NO2). Of the two, NO2 is the gas that causes us the most harm. It’s an irritant that can cause inflammation of our airways and can affect immune cells in the lungs. 

Over a prolonged period of time, it's thought that NO2 can affect how our lungs work.

CO (carbon monoxide) 

CO has always been lethal in an enclosed space. But Defra now warns that excessive CO increases the risk for people with existing diseases that affect delivery of oxygen to the heart or brain, such as angina.

PM (particulate matter) 

These are tiny particles of solid and liquid matter. From cars, PM comes from the exhaust as well as from tyres and brakes as they wear down. 

Particles are measured in nanometres, and some are small enough to pass through the pores in our lungs. PM has been linked to cardiovascular and respiratory disease. We hope to add PM measurements to our reviews and the tool above later in 2020.

NOx and PM 

There are a number of difficulties in trying to get an accurate figure of how many premature deaths that air pollution from NOx and PM cause, and it's impossible to separate the effects of each emission. But a 2018 report by the Committee on the Medical Effects of Air Pollutants (COMEAP) puts the UK figure as an equivalent of 28,000 to 36,000 premature deaths from NO2 from NOx and PM combined, or 328,000 to 416,000 life years.

Greenhouse gas

CO2 (carbon dioxide) 

CO2 is a greenhouse gas linked to climate change and rising global temperatures. In our tests, we’ve found that CO2 from cars is rising, not falling. 

See our guide to CO2 from cars for more information.

Euro emissions explained

The Euro emission standards to date are explained below – click on each drop-down arrow to read the details.

Each Euro standard has two introduction dates: an early one for new type approvals, and a second one for all remaining new cars on sale.

A new type approval is typically a new generation of car. For instance, the 7th generation (or Mk7) version of the popular VW Golf (2013-2020) was replaced by a new generation of VW Golf (2020-) in 2020.