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What Is The Purpose Of Emissions Test

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  • 19-07-2022
What Is The Purpose Of Emissions Test

Emissions test determines the overall level of motor vehicle air pollutants emitted fuel consumption, alongside general tests of effectiveness. The goal and purpose of your real driving emission test are to reduce the number of pollutants that are most dangerous to the environment and ozone.

Most old vehicles and new cars in the UK must get an MOT which often includes testing the vehicle's exhaust emissions using a meter. It is essential to have your vehicle's emissions tested to benefit the environment. It will allow you to get to grips with the level of pollution your vehicle produces. Pollutants affect the air quality of those around you and can negatively affect your health.

The particulates from standard combustion engines are soot by-products of fuel leading to dangerous respiratory issues. Carbon dioxide, as we know, is a greenhouse gas contributing to global warming and too much smoke emitted will add to this dangerous level in the air. It's paramount to do as much as possible to ensure your vehicle is reliable, safe to drive on the road and safe to sell to consumers. 

Emissions testing In the UK

MOT Testing

Many vehicles worldwide must have their exhaust emissions tested in formal MOT tests. Often professionals use a meter to test the overall condition.

It was introduced by the government and vehicle manufacturers that those car models using petrol before August 1975 and diesel vehicles before January 1980 must be tested by sight. If your vehicle or car continuously fails the MOT test, you must get the issues fixed immediately to obtain the MOT certificate.

The only vehicles that do not require emission tests during an MOT test include:

 The vehicles with fewer than four wheels

 The vehicles with 2-stroke engines

 The hybrid vehicles

 Those with quadricycles

 All hydrogen fuel cell vehicles

 Any electric vehicles

Roadside checks

Technicians will test your vehicle with a roadside check. If your vehicle is not up to standards and fails the check, you'll be issued a prohibition notice and feedback form, with 10 days left to fix the problem.

Those who fail to fix your car's issues within this timeframe will be prosecuted.

Suppose your vehicle breaches the legal emission limits seriously or has specific defects that make it particularly harmful to yourself or others road users, the notice will become more detrimental.

You may not be able to utilise your car or vehicle until it has been fully fixed and re-inspected. There are so many benefits that come with roadside checks, as they assure you of what requires checking or repairing. 

Air Quality Management Areas

Across the UK, numerous borough councils are regarded as Air Quality Management Areas. Your local council can offer a single test for your vehicle on and around the roadside and provide fixed penalties for those drivers that fail due to faulty vehicles with a lack of maintenance and care. 

How Are Vehicles Tested For Emissions?


Your vehicle examiner will start by taking your car through a standard MOT test. Before you enter the inspection hall or centre, all details of your car will be accurately checked and recorded on a computer. They'll first check the exhaust emissions of your vehicle. 

Diesel smoke test:

All diesel engines, buses, and heavy goods tend to emit smoke that mechanics will assess at this stage for its density. These tests are carried out by calibrated and approved smoke meters.

Other vehicles that emit smoke that will need to be tested include:

 Articulated Goods

 Road Construction Vehicles

 Road Construction Vehicles

 Tower Wagons

 Motor Tractors

 Display Vehicles

 Omnibuses and Play Buses

 Large Passenger Carrying Vehicles

 Breakdown Vehicles

Petrol exhaust test:

The test applies to taxis, trucks, minibuses, petrol-engined cars, ambulances with 12 passenger seating, and any vehicle up to or exceeds a design gross weight (DGW) of 3,500kg.

It also concerns petrol-engined / spark ignition vehicles with approximately four or more wheels.

At this stage in the process, they proceed with an under bonnet inspection that checks:

  • Registration plates and vehicle identification number (VIN)
  • Power-assisted steering
  • Engine transmission mounts
  • Mechanical brake components
  • Hydraulic, air and vacuum brake systems
  • Fuel system
  • Brake and steering fluids
  • The general condition of the vehicle
  • Vehicle structural integrity and construction


The second stage of your MOT test will then continue with a headlight alignment check. European headlamps tend to be the most prevalent type of headlamp that gets fitted onto a vehicle. They will check the aim on its dip beam for accuracy and brightness.

Often these headlamps come with an asymmetric dipped beam pattern; its cut-off on the right is distinctive and horizontal. It has a 'kick-up' towards its left side, a 15-degree wedge of light above that cut-off. You'll have no opportunities to adjust the alignment of your headlamp mid-test, so it is something you want to ensure is in the correct position beforehand. 


The third stage of the MOT process begins with what is known as a shock absorber test. Often your examiner will proceed with a break test, where the wheels of your vehicle are placed atop rollers with the brakes applied.

At this point, your technician or mechanic will inspect the driver's compartment and its significant effect on your car. 


At this stage, your examiner will position your car on its hoist so that it can have a complete body inspection. Underbody inspection often looks at all the items we've listed, and many involve your examiner taking your car on a short drive on the roads within or around the centre grounds to test it. Driving the car allows them to assess the governed speed and the car's performance when in motion.

The essential items they will search through and inspect include:

  • Electrical wiring
  • Body condition
  • Steering system and electronic stability control
  • Tyres
  • Road wheels and hubs
  • Suspension, wheel bearings and driveshafts
  • Shock absorbers
  • Oil leaks
  • Engine and transmission mounts
  • Transmission
  • Exhaust system including the catalytic converter
  • Mechanical brake components
  • Brake hydraulic, air and vacuum systems
  • Fuel system
  • Spare wheel and carrier
  • The general condition of your vehicle
  • Vehicle structural integrity and construction

The list above presents the many thoroughly checked items throughout the inspection stage. It looks relatively exhaustive, but overall its consists of the main items and aspects of your car that require sufficient checking.

For those that require further information on what is checked, contact your local shop or mechanics via their phone number or visit their website.

At the end of your test, centre management may randomly select your vehicle for a re-check as part of quality control. You want to ensure your car maintains its high standards so that you can successfully pass and meet the regulations.

Once your test and inspection have been officially completed, your examiner will then tell you whether or not you have passed or failed your MOT test.

If you successfully pass your MOT test, your examiner will present you with the MOT certificate that assures you your car is safe to be driven on the road.

Drivers no longer receive MOT disc's that you can display on your windscreen as proof.

Your MOT certificate can list minor defects or faults that your car may have; these faults or defects do not massively affect the environment or the safety of those on the road with you.

However, it is there to let officers or other mechanics know about the condition of your car. Such allows you to address the faults in good time before they develop into more severe issues.  In a way, they remind you and others that you are due services. However, if your vehicle does not pass, your examiner will provide you with a record sheet that fully details each fault you must have repaired before any retest.

After a failed attempt, you can book any retests at a more reduced fee within the following 21 days of your previous test. Retests at lower fees will often occur 60 days after your original test. If you miss the deadline, you'll be expected to pay the entire cost again.

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