MOT tester checking car during an MOT test


3 mins

Why MOT tests include an exhaust emission test

December 12, 2023

In the UK, any vehicle older than three years will need an MOT test conducted yearly. The test ensures that a vehicle meets road safety and environmental standards.

An important part of an MOT is the exhaust emissions test, and in 2020 alone 1.3 million vehicles failed their MOT because they failed the exhaust emissions test. If you’re wondering why MOT tests include an exhaust emission test, keep reading – we’ve broken this down for you!

What is an exhaust emissions test?

An exhaust emissions test ensures the levels of pollutants emitted from your car exhaust are within legal limits.

The test is compulsory for all diesel, petrol and gas-powered vehicles with four or more wheels:

  • For petrol or gas-powered vehicles first used before 1 August 1975, a visual test is applied
  • For petrol or gas-powered vehicles first used on or after 1 August 1975, a metered test is applied
  • For diesel vehicles first used before 1 August 1979, a visual test is applied
  • For diesel vehicles first used on or after 1 August 1979, a metered smoke test is applied

Why do MOT tests include a strict exhaust emission test?

The UK is actively working to improve air quality – which is evident with the various Clean Air Zones launching across the country.

Emission tests are a quick way to ensure that vehicles are complying with government set standards.

There are many reasons why your car may emit higher emissions than usual, for example:

  • A simple blocked air filter
  • A fault in the Engine Management Unit causing the Engine Management Light to turn on
  • Severe engine damage (blown cylinder gasket, worn cylinder rings or damaged pistons) causing excessive oil consumption
  • Defective catalytic convertor causing an increase in dangerous emissions being released from a vehicle

If pollution produced by the engine is higher than expected, it indicates that your vehicle is not operating correctly and can cause you to fail your annual MOT test.

To avoid failing an MOT because of high emissions, good car maintenance and regular servicing is encouraged. Make sure to have regular oil checks and are dealing with issues such as exhaust repairs quickly.

What is the procedure for emissions testing?

1. Preliminary checks (all vehicles)

As a general precaution, motorists are advised to check the following things on their vehicle:

  • There is enough engine oil
  • The coolant level is correctly topped-up
  • There is enough fuel to carry out the test

Before proceeding, the tester will also check that the vehicle has sufficient oil and fuel to complete the test.

2. Visual inspection (all vehicles)

Once the preliminary checks have been completed, the tester will:

  • Raise the engine speed to around 2,500rpm for approximately 20 seconds and will then return it back to its natural idle speed
  • Once stabilised, the tester will assess the smoke emitted from the tailpipe
  • If the engine is emitting dense blue or clearly visible black smoke, the vehicle will fail the test

3. Standard emission test (petrol vehicles only)

Petrol vehicles will also be checked using an analyser to assess the concentration of carbon monoxide (CO) and hydrocarbons (HC).

The first test consists of checking the emissions at an engine speed of 2,500rpm – 3,000rpm. During this test, CO must be at or less than 0.2%.

The second test consists of checking the emissions at an engine speed of 450rpm – 1,500rpm. During this test, CO must be at or less than 0.3%

4. Metered smoke test (diesel vehicles only)

Before the smoke test, the tester will ensure that the engine is functioning correctly by raising the engine speed to 2,500rpm. The engine speed will then be slowly increased to the maximum (around 3,500rpm – 4,500rpm) to check the operation of the fuel pump.

  • If the engine speed stabilises at its maximum speed, the engine will be returned to the idle speed and will move on to the next part of the test
  • If it is not able to stabilise, the engine will be returned to the idle speed and the vehicle will fail the test

For all vehicles that pass the initial speed test, the tester will prepare a smoke meter and insert the sampling probe into the exhaust tailpipe.

There are also various other pre-checks you can do yourself before sending your car for its MOT to minimise the risk of failing. You can read them here.

What’s the best way to book an MOT?

Conducted all the pre-checks and are ready to send your car for its MOT? We’ve made it quick and easy for you to book a test!

Simply open the Caura app and tap on ‘Book Garage’.

Remember, it’s not just MOTs, servicing and repairs we make simple, it’s all your car admin. You can renew car tax in Caura in under 30 seconds and get speedy insurance quotes from market-leading insurers in under a minute!

We’ll remind you when each of these renewals are coming up, and all receipts are stored in the app, ready for you whenever you may need them.

MOT, servicing and repair booked in a couple of taps in the Caura app

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