Why do Exhaust Emissions Tests Exist on MOTs?

Jack Dreyer | Monday 7th August 2023 9:00am

A car exhaust emits dark fumes. Text reads 'Why do Exhaust Emissions Tests Exist?'.

If you’ve had an MOT in the past five years, you’ll have come across the exhaust emissions test. 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 roadside.

But why was this test added to the MOT assessment 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.

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

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-recognized 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 “lamba” (?) 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.

A technician looks under a car and checks an exhaust during an MOT.

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 global warming, a phenomenon currently wreaking havoc on our planet in the form of climate change.

So, what are the current carbon emissions produced by the vehicles on Britain’s roads?

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 in this year.

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 significant 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

So, 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.

And it’s working! In recent years, since the introduction of the emissions test (alongside other measures), there have been significant reductions in NOX levels as drivers are forced to take greater responsibility and accountability for the cleanliness of their vehicles by doing things like exhaust checks.

A probe is used to check an exhaust pipe for emissions.

Time for your MOT?

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

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

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