Why Are There Emissions Tests On MOTs?

If you’ve had an MOT in the past five years, you’ll have come across the exhaust emissions test. It’s designed to test whether the gases emitted from a car’s exhaust are below the levels stipulated by both national and European legislation; legally, you cannot get an MOT certificate if your vehicle’s exhaust emissions are too high.

Every testing station or garage that offers MOTs is required to test a car’s exhaust emissions - at least for every domestic vehicle over three years old. As well as this, though, your vehicle’s exhaust emissions can also be tested at the roadside.

But why was this test added to the MOT in the first place? Read on to find out.

The exhaust emissions test explained

Which vehicles don't need emissions testing?

Any petrol vehicle used before August 1975 - or diesel vehicle used before January 1980 – aren’t subject to an emissions test via the usual meter method. Instead, these vehicles are tested by sight, looking at what comes out of the exhaust. This is, understandably, a fairly inaccurate measure – but the negatives of this are outweighed by the fact that it applies to very few vehicles (i.e. there aren’t that many vehicles built before 1980 still driving on British roads).

All other vehicles are required to pass an emissions test if they want to pass their MOT, apart from the following:

• Vehicles with less than four wheels
• Vehicles with 2-stroke engines
• Electric vehicles
• Hybrid vehicles

How does the exhaust emissions test work?

Simply put, the exhaust emissions test is an assessment in which specialist equipment is used to measure the gases a car’s exhaust produces, and compare them to guidelines. Usually, the following steps take place:

• You book your vehicle in for an MOT at your local Kwik Fit or MOT testing centre.
• The MOT begins and, once the exhaust testing section starts, a probe (also known as an exhaust gas analyser) is placed in the tailpipe of your vehicle.
• This instrument should be approved by a nationally-recognised body and calibrated using a CRS (Certified Reference Standard) produced in accordance with ISO17025.
• The vehicle is turned on, and four core exhaust gases are measured:
• Oxygen (O2)
• Carbon Monoxide (CO)
• Carbon Dioxide (CO2)
• Hydrocarbons (HC)
• As well as these gases, a metric known as “lambda” is calculated. This is a measure of the air-to-fuel ratio in the gas.
• As a result, an overall value of the efficiency of the engine is given.

Increasingly for drivers, an MOT is now not the only place they will encounter an emissions test. Some boroughs and local constituencies are ‘Air Quality Management Areas’, which means that the council can test vehicles for their emissions at the roadside – even issuing fixed penalties to drivers whose vehicles fail. So, it is essential to stay vigilant with your car’s emissions.

Common reasons for emissions test failures

Petrol cars

• Faulty catalytic converter increasing CO and HC levels.
• Worn spark plugs affecting combustion.
• Failing oxygen sensors disrupting fuel-air balance.
• Engine misfires or running too rich causing incomplete combustion.

Diesel cars

• Blocked or failing diesel particulate filter (DPF) leading to excess smoke.
• Frequent short journeys preventing DPF regeneration.
• Faulty fuel injectors reducing efficiency.
• Turbocharger issues affecting performance.
• EGR valve faults increase particulate output.

For both petrol and diesel cars, general maintenance plays a crucial role. The following can all contribute to higher emissions:

• Engine warning lights
• Clogged air filters
• Exhaust leaks
• Poor-quality fuel
• Missed servicing

Keeping the vehicle well maintained and addressing issues early can significantly improve the chances of passing an MOT emissions test.

Why do exhaust emissions tests exist?

Over the past ten years, the UK government has moved closer and closer to their goal of achieving Net Zero Emissions by 2050, owing to a clearly-laid out Carbon Reduction Policy. A huge part of meeting this target has been directly to do with reducing the amount of greenhouse gases (GHGs) produced by transport on Britain’s roads. Why? Because, in 2016, transport rose to be the largest emitting sector in Britain’s excessive greenhouse gas emissions, and these gases directly contribute to climate change.

Carbon emissions and our cars

According to the Department for Energy Security and Net Zero, in 2020, the UK produced 406 MtCO2e (that’s million tonnes of CO2 equivalent) of greenhouse gas emissions. Of this figure, UK transport was responsible for 99 MtCO2e (that’s a whopping 24% of emissions).

The majority of these emissions (91%) came from road vehicles – more specifically, cars and taxis, which produced 89 MtCO2e.

Air pollution

Alongside CO2, the exhausts on our cars also produce gases and other substances that, although having a less significant greenhouse gas effect, can have health consequences.

Namely, these take the form of Nitrogen Oxides (NOX) and particulate matter (PM). According to the National Atmospheric Emissions Inventory (or NAEI), a third of the nitrogen oxides and 14% of particulate matter produced by the UK came from the transport sector in 2020.

Cracking down on exhaust emissions

When taking the statistics about greenhouse gases and air pollution above into consideration, it’s easy to see why the transport sector has come under so much scrutiny from the government with its laws and reforms.

The exhaust emissions testing in MOTs is just one of these reforms, alongside:

• The encouragement of UK citizens to use public transport or bicycles.
• The promotion of hybrid and electric vehicles.
• Laws stating all newbuild areas must have EV chargers.
• Rules around EV charging points.
• The introduction and expansion of Low and Ultra Low Emissions Zones into major cities.
• Clean Air Financial Support Schemes in major cities with ULEZ regulations to help drivers upgrade their vehicles.
• And more.

In recent years, since the introduction of the emissions test (alongside other measures), there have been significant reductions in NOX levels - which shows that these reforms are working. 

FAQs

What's the difference between diesel and petrol emissions tests in an MOT?

Petrol and diesel cars are tested differently because their engines produce different pollutants. Petrol cars are tested using a gas analyser that measures carbon monoxide, hydrocarbons, and the fuel-air ratio. Diesel cars are tested using a smoke opacity test, which measures how much soot is emitted from the exhaust.

Do diesel cars have stricter MOT emissions rules?

They do, particularly for newer diesel vehicles. In fact, diesel vehicles are gradually being phased out by many manufacturers, specifically because of the stricter emissions rules.

Why do diesel cars often fail MOT emissions tests?

Failures are commonly linked to excessive soot, a blocked or faulty DPF, injector or turbo issues, poor maintenance, or a build-up from short journeys (which tends to clog the filters).

Can I drive my car if it fails the emissions test?

No – if your car fails its MOT due to emissions, it is not road legal (unless driving to a pre-booked repair or test). Repairs are required before a valid MOT certificate can be issued.

Will emissions rules get stricter in the future?

It’s most likely that emissions standards will continue to evolve – and we’ll continue to see a push for drivers and manufacturers to ensure that cars have as little impact on the planet as possible.

Time for your MOT?

If your car is due its MOT soon, why not book in for a service and MOT appointment at Kwik Fit to help make sure everything’s working as it should be. Or, if you’re not sure when your MOT is, now’s a great time to sign up to our reminder service.

If you have any more questions about exhaust emissions, or any of your car’s systems, get in touch with your local Kwik Fit today.