What To Do If Your Car Fails Its MOT On Emissions

Protecting the environment is a big deal these days, but if your car fails the emissions portion of the MOT test, it’ll be impacting your own health too.

---

What Is An Emissions Test For A Car?

Part of the annual MOT test, an emissions test is something your petrol or diesel car will be put through to ensure it’s not spitting out polluting fumes at an unacceptable level. These tests aren’t expecting your car to be completely free of pollution; the levels just need to be below a maximum measurement.

How To Clean Up Your Emissions From Your Car

Think your car will fail an MOT test on its emissions result? There are ways to reduce the levels that your car produces. Some of them are actions you can take whilst driving, whereas others could help your car pass that dreaded test when the exhaust probe comes out:

• Add a cleaning agent directly into your vehicle’s fuel tank.
• Use better quality fuel the next time you’re at the pump.
• Change your oil to prevent it from becoming inefficient.
• Change your air filter so that air can still circulate properly around the vehicle.
• Use the correct tyre pressure to ensure a smooth ride.
• Turn off your air con so that your car isn’t using as much power.
• Drive more smoothly instead of accelerating hard and hitting high speeds.

Which Cars Are Emissions Tested On An MOT?

Not all vehicles are tested for emissions, and in fact even ones that are can sometimes be tested in different ways.

If your vehicle is a hybrid or fully-electric, or even has only a 2-stroke engine, then it will not be emissions tested. After all, an EV (electric vehicle) doesn’t have an exhaust to produce fumes from!

Very old cars are emissions tested, but the method of doing so is very rudimentary. Petrol cars used before 1975 and diesel cars used before 1980 aren’t assessed using any kind of modern tech. No, the MOT technician will simply look at the fumes coming from the car’s exhaust and give it a pass or fail. They were simpler times…

How Are Car Emissions Tested On An MOT?

We mentioned in another section that cars from the ’70s or earlier were tested using the naked eye to see if the exhaust fumes looked a bit nasty. Well, these days it’s more technical than that.

A device called an exhaust gas analyser – a kind of probe – is inserted into the exhaust tail pipe of your car. When switched on with the car running, it will measure the gases that your vehicle is emitting.

What Emissions Are Being Checked?

This is where it gets a bit scientific. It’s not just a case of ‘smoke = bad’ at all. The levels of four gases are measured by the device that’s inserted into your exhaust. They are:

• Oxygen
• Carbon monoxide
• Carbon dioxide
• Hydrocarbons

Of course, carbon monoxide is bad for us, so you don’t want your car to be putting too much of that out into the air. The big problem, though, is the carbon dioxide. One of the most well-known greenhouse gases, carbon dioxide is a huge contributor to global warming.

What Is The Emissions Limit For An MOT Test?

The gov.uk website lists the following as the requirements for your car to pass the basic emissions test included in an MOT test:

• Carbon monoxide – up to 0.2% while engine running at 2,500rpm to 3,000rpm
• Hydrocarbons – up to 200ppm while engine running at 2,500rpm to 3,000rpm
• Lambda* – between 0.97 and 1.03 while engine running at 2,500rpm to 3,000rpm
• Carbon monoxide – up to 0.3% while engine running at 450rpm to 1,500rpm

*Lambda refers to the ratio of air-to-fuel in the car’s combustion chamber. A lambda sensor measures the oxygen in your emissions.

Emissions FAQs

How Do Emissions Cleaning Agents Work?

Cleaning agents, like Redex or Adblue (other brands are available), work to clean up your fuel and exhaust systems by breaking down solid deposits of carbon and remove gases from the fuel before it is used and expelled from your car’s exhaust.

Typically, these are added to the fuel by hand afterwards, but some petrol stations will offer fuels that already contain these additives. For example, it’s fairly common for petrol stations to have a pump that offers Adblue for diesel drivers (unleaded cars don’t need Adblue).

Will Changing Oil Lower My Emissions?

It can, yes. Old oil will have built up plenty of hydrocarbons which are bad for the environment and need to be kept below a certain level. So, swapping old oil for new will remove those hydrocarbons and therefore make your car more environmentally friendly.

How Much Does It Cost To Fix An Emissions Test Failure?

This depends on what’s causing the problem. Minor fault repairs can set you back around £200, but if you’ve got a problem with your emission-reducing catalytic converter, expect a much higher bill. Cat converters are notoriously valuable – that’s why they’re stolen so often! If you need a new one, you could have to pay out up to £1,000!

---

If your repair bill is simply too high to justify, sell your car with Scrap Car Comparison instead. We’ll quote you a price for your car in just 30 seconds, and all we need is your vehicle reg & postcode to do so! An MOT failure might mean the end for your car, but it doesn’t need to leave you out of pocket. Get a quote now – free collection of your car is included for every customer!

---

Or, visit our MOT failure hub for guides on everything related to MOT, and how to navigate your options after a fail.

---