Why Do MOT Tests Include an Exhaust Emission Test?
Ellie Dyer-Brown, 2 months ago
5 min read
- MOT
- Car ownership
FixMyCar explains why exhaust emissions are tested during an MOT.
MOTs are a fact of life for UK motorists, though many of us are unsure what is tested and why. This guide explains the importance of the MOT emissions test, which is carried out on vehicles with petrol and diesel engines.
Contents
What is an exhaust emission test?
Why do MOT tests include an exhaust emission test?
What happens during an emission test?
The importance of MOTs
Cars over the age of three are required by law to have an annual MOT test for safety and environmental reasons. The test ensures that vehicles are roadworthy and don’t cause too much pollution, and it helps promote a high car maintenance standard. You can learn more about what is checked in an MOT here.
What is an exhaust emission test?
Emission tests are used to measure the level of pollution that comes out of the exhaust. For a car to pass this part of the test, the meter reading must be within the following limits:
Carbon dioxide (CO2) up to 0.2% at fast idle (2,500rpm to 3,000rpm)
Hydrocarbons (HC) up to 200ppm at fast idle
Lambda between 0.97 and 1.03 at fast idle
Carbon monoxide (CO) up to 0.3% at idle (450rpm to 1,500rpm)
If the level is too high, your car will fail its MOT and can’t be driven until the issue is fixed.
Why do MOT tests include an exhaust emission test?
Climate change is one of the biggest threats we face today, along with health concerns from increased pollution. These are two main reasons an emission test is included in your car’s annual MOT.
1. Environmental impact
Emission tests are helpful for monitoring and reducing the environmental impact of cars, which has a significant role in climate change.
2. Health impact
Pollutants aren’t just harmful to the climate; they’re also bad for our health, causing respiratory conditions and cancer.
3. Legislative compliance
Strict environmental legislation by national governing bodies must be met, and emissions tests are one way to achieve this.
4. Vehicle performance and maintenance
Testing a car’s emissions can provide helpful information about potential mechanical issues. For instance, high emissions could indicate a faulty catalytic converter. Identifying problems early during an MOT test allows them to be fixed before they pose a risk to people’s safety or health.
What happens during an emission test?
1. Preliminary checks
Before your MOT, ensure your car has enough fuel, engine oil and coolant.
The tester will also check whether the vehicle has enough oil and fuel.
2. Visual inspection (all vehicles)
Next, the tester will:
Increase the engine speed to 2,500rpm for about 20 seconds before letting it return to its natural idle speed.
Assess the smoke coming from the exhaust.
The vehicle will fail if the exhaust produces visible black or dense blue smoke.
3. Standard emission test (petrol vehicles)
The emissions of petrol vehicles are checked using an analyser to assess the concentration of hydrocarbons (HC) and carbon monoxide (CO).
Two tests are conducted, one at a high engine speed and one at a normal idle speed.
4. Metered smoke test (diesel vehicles)
First, the tester will make sure the engine is working correctly by increasing the speed to 2,500rpm and then slowly raising it to the maximum revs. This checks the operation of the fuel pump.
If the engine speed stabilises, the next part of the test can be completed. If it doesn’t, the vehicle will fail the test.
Finally, a sampling probe from the smoke meter is put in the exhaust pipe.
You can learn more about the ins and outs of MOT emission tests on the government website.
How to reduce your car’s emissions
Here are our top five ways to reduce your car’s emissions:
Use a cleaning agent in the fuel system.
Buy premium fuel designed to reduce emissions.
Maintain the correct tyre pressure.
Change your engine oil regularly for optimum efficiency.
Avoid using the air conditioning when possible.
Check out this guide for extra advice on how to make your vehicle more eco-friendly.
What’s the best way to book an MOT?
Booking your MOT with FixMyCar is easy. Enter your registration number, and we’ll send you quotes from trusted local garages so that you can compare and book online.
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Written by Ellie
Ellie is FixMyCar's Content Writer. She has over three years of experience writing about cars and regularly collaborates with automotive experts to provide trustworthy advice for drivers that is easy understand. Her work has been featured in Yahoo! Finance, iNews, The Daily Express and The Sun. She has a BA in English literature and an MA in creative writing from Durham University. Outside of work, Ellie follows F1 and eagerly awaits Ferrari's next era of dominance in the sport. She drives a Suzuki Swift.