How to reduce car emissions for an MOT

    cars in traffic

    A core part of the Ministry of Transport (MOT) vehicle exam is evaluating emissions output. As with the safety and vehicle functionality components of the MOT exam, all passenger cars and vans must meet the MOT’s emissions standards to legally drive on UK roads. 

    From routine maintenance to eco-friendly driving habits, there are a variety of ways to lower your vehicle’s environmental impact, ensure compliance with emission standards, and pass your annual MOT exam. Failure to do so can jeopardise your ability to legally drive in the UK, and may impact the resale price of your vehicle if and when you choose to sell your car.   

    Read on for tips and tricks to vehicle emissions reduction.

    Vehicle emissions and the MOT

    The purpose of the MOT is to check the safety, roadworthiness, and emissions of the vehicles on UK roads. Vehicles from small motorcycles under 200cc to the biggest lorries are required to take – and pass – this annual exam to continue driving legally.

    As the MOT examines the environmental impact of cars driving across the country, exhaust emissions play a crucial role in the MOT test. Research has shown that high vehicle emissions levels directly contribute to pollution, degrade air quality, and pose risks to public health. The MOT’s stringent emissions standards are designed to limit car emissions in the name of community and environmental health.

    In addition to the MOT, Clean Air Zones (CAZ) and Low Emissions Zones (LEZ), such as London’s Ultra Low Emissions Zone (ULEZ), across the country are working to decrease emissions levels via fines and penalties. 

    Factors affecting vehicle emissions levels

    car exhaust with fumes coming out of it
    Emissions are a crucial aspect of the MOT exam.

    🚘 Engine and fuel type

    Your car’s engine impacts how much exhaust it produces. Both petrol and diesel combustion engines produce emissions as part of their normal operation. Diesel engines emit more nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM) than petrol engines. In general, petrol engines produce fewer emissions due to cleaner combustion.

    All vehicles must meet Euro 6 emissions standards as a part of the MOT exam. However, keep in mind that these standards are different for diesel and petrol cars: 

    Euro 6 emissions limit: no more than 0.50 CO, 0.080 NOx, 0.005 PM
    Emissions reduction tools: advanced particulate filters and selective catalytic reduction (SCR) systems; retrofitting older diesel vehicles; promoting cleaner diesel blends
    Euro 6 emissions limit: no more than 1.0 CO, 0.060 NOx, 0.005 PM
    Emissions reduction tools: direct injection, variable valve timing, and turbocharging

    Not all fuels are made equal. In terms of quality, higher quality fuels result in cleaner combustion and lower emissions. Premium fuels contain fewer impurities, resulting in cleaner combustion and reduced emissions. Additives further optimise fuel combustion, reducing carbon buildup and pollutants emitted by the vehicle. Conversely, poor quality fuels can increase emissions output and environmental harm.

    🚘 Vehicle age, weight, and condition

    Older vehicles tend to emit more pollutants as they do not possess the same emissions technology as newer cars. Because of this, vehicles registered before 1 March 2001 are likely to be subject to further emissions regulations, especially as they are likely non-compliant with the UK’s Clean Air Zones (CAZs).

    Well-maintained cars also generally emit fewer pollutants than poorly maintained ones, making vehicle upkeep an crucial part of preparing for your MOT exam.

    Additionally, larger and heavier vehicles generally produce more emissions than lighter ones due to their size and weight.

    🚘 Driving habits

    Aggressive driving, excessive idling, and rapid acceleration can increase vehicle emissions and also cause engine damage in the long run. Instead, practise eco-friendly driving practices like smooth acceleration and maintaining steady speeds to reduce emissions.

    🚘 Engine efficiency and technology

    Is your car eco-savvy? Many modern engines have advanced emission control systems designed to emit fewer pollutants. Technologies like catalytic converters and exhaust gas recirculation also help reduce emissions.

    As emission control systems can significantly reduce harmful pollutants, regular maintenance is crucial.

    🚘 Alternative fuels and powertrains

    Electric, hybrid, and hydrogen-powered vehicles produce zero emissions during operation. Switching to alternative fuel vehicles (as available for your vehicle) reduces reliance on fossil fuels and decreases emissions – helping you breathe a little easier and breeze through the MOT exam.

    🚘 Environmental regulations

    Stringent emission standards drive both manufacturers to produce cleaner vehicles and consumers to look for more environmentally-friendly cars and vans. In addition to the MOT, CAZs and LEZs across the UK are enforcing environmental standards and reducing car pollution in city centres through non-compliance penalties and fines. The expansion of these zones is also helping to shift consumer preferences for more eco-conscious transport.  

    Additionally, the UK government is working towards a ban on the sale of new petrol and diesel cars and vans in 2035. This effort to promote zero-emissions transportation is further impacting the car-buying decisions of consumers nationwide. 

    Cleaner emissions with regular car maintenance

    Routine vehicle maintenance can help reduce emissions levels.

    Regular car servicing helps keep your vehicle in top shape – and can decrease your emissions output. Inspecting your engine, exhaust system, emission control systems, and other core car components on a regular basis helps detect issues that could lead to increased emissions, such as faulty oxygen sensors or clogged air filters. 

    Identifying and addressing these issues promptly keeps your vehicle running safely and efficiently, while also minimising its environmental footprint.

    Routine oil changes are also essential for maintaining engine health and reducing unnecessary emissions. Old or dirty oil can lead to increased friction and wear, resulting in reduced fuel efficiency and higher emissions. Choosing the right oil, such as low-viscosity synthetic oils, can further improve engine performance. 

    Finally, using oils with additives designed to clean the engine and reduce deposits can help maintain optimal emission levels over time. By prioritising regular oil changes and selecting the appropriate oil for your vehicle, you can contribute to cleaner air and a healthier environment.

    Vehicle components combatting emissions

    Catalytic converter

    Many cars and vans feature speciality components that help reduce their emissions. Catalytic converters decrease vehicle emissions by converting toxic gases and pollutants into less harmful substances. These exhaust emission control devices transform exhaust from both diesel and petrol internal combustion engines into safer byproducts, such as steam. Catalytic converters have been mandatory in new cars and vans in the EU since the 1990s.

    Diesel Particulate Filter and Selective Catalytic Reduction systems

    Diesel Particulate Filters (DPFs) and Selective Catalytic Reduction (SCR) systems also play a role in reducing emissions from diesel engines. DPFs trap and burn particulate matter, while SCRs use a catalyst to convert harmful NOx into harmless nitrogen and water vapour. 


    Maintaining the correct tyre pressure is crucial for vehicle performance and environmental impact, especially on UK roads. Properly inflated tyres reduce rolling resistance, allowing the vehicle to move more efficiently. With reduced rolling resistance, the engine doesn’t have to work as hard to propel the car forward, resulting in improved fuel efficiency.

    Over time, driving with under-inflated tyres can contribute to increased carbon dioxide (CO2) emissions, which are detrimental to the environment and public health. Ensuring that tyres are properly inflated helps the engine operate more efficiently, emitting fewer pollutants into the atmosphere.

    Lifestyle changes 

    To some degree, how you use your car also shapes its emissions output. The following driving habits are all proven to lower vehicle emissions in the long run: 

    Smooth acceleration and deceleration – Avoiding rapid changes in speed helps reduce unnecessarily exhaust output. Practising smooth driving and maintaining constant speeds as much as possible will also protect against wear-and-tear on essential vehicle components. 

    Reduce idling time – Turn your engine off if your vehicle is stationary for over a few minutes to save fuel and reduce emissions output. 

    Plan efficient routes – Choose routes with less traffic congestion and fewer stops. Utilise GPS navigation for real-time traffic updates.

    Combine trips and carpool – Combine errands into one trip and consider carpooling to reduce the number of vehicles on the road. 

    Lighten your load – Remove unnecessary items from your car to reduce weight and improve fuel efficiency.

    Eco-friendly driving practises – Consider eco-friendly driving features to minimise emissions. Utilise eco mode and regenerative braking when possible.

    Regular vehicle maintenance – Keep your car well-maintained with regular servicing to ensure optimal performance and lower emissions.


    What causes emissions failure on MOT?

    Common causes of emissions failure on MOT include faulty oxygen sensors, dirty air filters, malfunctioning catalytic converters, engine misfires, excessive oil consumption, and exhaust leaks. These issues can lead to higher emissions levels that exceed MOT standards.

    If you have any concerns, consult with a mechanic about your vehicle’s emissions output before your MOT exam.

    Where do I find the CO2 emissions rating for my car?

    Your vehicle’s Euro standard emissions rating is located in your V5C logbook. You can also find the fuel consumption and emissions information for new and used cars on TotalCarCheck. Simply enter your car registration to access a huge range of information about your vehicle, including CO2 emissions. 

    Why are my car emissions so high?

    High car emissions can result from faulty engine components like oxygen sensors or catalytic converters, dirty air filters or fuel injectors, engine misfires, running rich fuel mixtures, or worn-out spark plugs. These factors can lead to incomplete combustion and increased emission levels. 

    Have a qualified mechanic conduct a full vehicle assessment to identify the causes of additional emissions output and make repairs as needed. 

    How much does it cost to fix emission problems?

    For minor issues such as replacing oxygen sensors or cleaning air filters, emission repairs can cost between £100 and £200. However, larger repairs such as catalytic converter replacement can range from £500 to over £1,000, depending on the extent of damage and the vehicle model. Consulting a mechanic is crucial for accurate estimates.

    How can I check my car’s MOT status? 

    With Motorway’s new MOT Checker, it’s never been easier to know the status of your vehicle’s MOT.

    Simply enter your vehicle registration number into our free tool and tap the ‘Check MOT’ button. You’ll get an instant response saying if your MOT is valid and giving you a countdown until your certificate runs out.  

    With the MOT Checker, you can also explore your vehicle’s full MOT history and, if your car possesses a valid MOT certificate, set reminders for when to schedule your next exam. 

    Can you sell a car without an MOT?

    Yes, you can sell a car if it doesn’t have a valid MOT. However, this will be much more difficult!

    Lacking an MOT certificate will likely reduce both your potential buyer pool and your vehicle’s sales price as buyers are shopping for safe transport. Since their roadworthiness has not been verified, cars that do not have valid MOT certificates carry an additional element of risk and may seem like a riskier investment. Similarly, cars that have failed their MOT are more likely to be classified as Cat D write-offs and could be dangerous! 

    If your car fails the MOT test and you’re looking to get rid of it, another option could be to sell it for scrap

    Is it time to sell your car?

    Want to learn more about owning, maintaining, and selling your car
    Check out more of our guides here, covering everything from finding buyers, to negotiating a good price, and completing payment safely. And don’t miss our new MOT checker tool, which lets you check your car’s MOT status and history instantly – for free!