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How to reduce diesel emissions for an MOT

Kwik Fit | Tuesday 29th October 2019 11:29am

car exhaust emitting white smoke

Taking your car for an MOT test can be stressful - will your vehicle be road legal for another year or will it be time to say goodbye?

Obviously, you want your vehicle to pass the MOT test. You may have completed a thorough check of the headlights, oil levels and tyre tread, but you may not have thought about the emissions that your diesel engine is producing. Emissions that are deemed too high could result in an MOT failure, particularly as there has been a crackdown on acceptable car emissions levels since May 2018.

Why do MOT tests include a strict exhaust emissions test?

There’s a large focus, particularly within Europe, to reduce pollution levels that are produced by diesel engines. This is why certain limits were introduced to ensure that cars aren’t creating excessive emissions that could be harmful to our planet. In May 2018 the MOT emissions test was altered, a change that was thought to be the biggest adjustment to the MOT test for 20 years.

The emissions test measures the amount and the concentration of carbon monoxide and hydrocarbons that are being released via the exhaust pipe. Diesel engines also produce nitrogen oxides (NoX), something that the government is also trying to reduce. Older diesel cars are more likely to produce a higher level of emissions and therefore could fail an MOT test for this reason.

An MOT test makes use of a scale that determines whether your vehicle is safe to drive. Each item that is checked will be marked as ‘Dangerous’, ‘Major’, ‘Minor’, ‘Advisory’ and ‘Pass’. Any items that come under the ‘Major’ or ‘Dangerous category will be a fail. A ‘Dangerous’ result is defined as ‘A direct and immediate risk to road safety or has a serious impact on the environment’.

How to reduce emissions on a diesel car

If you’re worried about your diesel car passing its MOT, there may be some steps that you can take to reduce its emission levels.

Install a Diesel Particulate Filter (DPF)

A DPF catches exhaust soot and can help to reduce emissions from diesel cars. The soot that becomes trapped has nowhere to go, so the filter does have to be cleaned or emptied every now and then to make sure that it can continue to work efficiently. This means that the filter can also help to reduce the thick black smoke that you sometimes see coming out of an engine’s exhaust.

Any diesel cars in the EU that were manufactured after 2011 will already have a DPF installed. If your diesel vehicle was made prior to 2011, you could have one installed to ensure that your car passes its emissions test.

It is illegal to remove your DPF and doing so could result in a fine of up to £1,000. It could also make your car insurance invalid.

Install a Selective Catalytic Reduction (SCR)

An SCR can convert your exhaust gases from NoX to nitrogen and water vapour, therefore reducing your vehicle’s emissions. It works by adding ammonia to the exhaust gases. When the ammonia and NoX are combined in the catalytic converter, the NoX turns to nitrogen and water vapour. These are the substances that will exit via your vehicle’s exhaust pipe.

This type of system can be expensive to install and may only be worth doing in lorries and buses, particularly as the system is quite big and may not fit in a standard car. However, it’s still an option that may be worth looking into if you’re worried about the MOT test.

Check your tyre pressure

Correct tyre pressure is more important than many drivers think. Having under-inflated tyres could lead to low-precision steering, increased fuel usage, a higher risk of aquaplaning in wet conditions and extended braking distances. All of these things are dangerous and could lead to an accident.

But under-inflated tyres could also be increasing your vehicle’s emissions. A tyre that doesn’t have enough air in it has to make more turns in order for your vehicle to move. Therefore, it’s more effort for the engine. This additional effort can have an impact on your car’s emissions and its fuel efficiency.

You should make sure that your tyres are at the correct pressure. We’d recommend checking them once a month and filling them up to the manufacturer’s recommended pressure if they’re low.

Use premium fuel

Using premium fuel could lower the emissions in both a petrol or diesel engine. Premium diesel usually contains added chemicals that work to shift build-ups of soot and other dirt or debris. Although it’s more expensive, it will work to clean your car’s engine, which could make it more efficient and reduce its emissions.


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