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, and your wallet too.

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What Happens If Your Car Fails Its MOT Emissions Test?

Failing the emissions portion will result in your car also failing its overall MOT test. Don’t panic – passing this test isn’t too difficult first time round, and repairs can usually be made to get the vehicle through a retest.

The test can actually be broken up into several stages. If your car passes at the first stage, you’re good to go and don’t need to worry. But, if your vehicle fails stage one, it’ll be put through to a full emissions test which is more rigorous. This test involves a lot of engine revving and can take up to four minutes.

Fail that test and unfortunately it’s game over. Your car has failed its overall MOT test. You’ll receive a print-out of the result and be informed of the reason for failure, with advice on how to fix the problems so that you can get it ready for a retest. This advice will differ depending on the problem, and could be as simple as using fuel additives to something as complex as an expensive mechanical repair job.

Diesel Cars

Diesel cars are tested in a similar way, but their smoke test (explained here) can be tested a further six times if the initial test fails!

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!

Classic Cars

MOTs work a bit different for classic motors. In fact, your old car might not need to undergo an MOT test at all!

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.

Diesel Cars

Diesel cars will initially undergo something known as a ‘smoke test’, which is exactly what it sounds like. A separate device will be used to measure the opacity of the smoke between two laser points. This will give a pass or a fail reading.

It’s not unheard of for a technician to fail the vehicle simply by looking at the smoke, because smoke that’s really bad will actually damage the device!

Petrol Cars

Petrol cars don’t go through the smoke test, but instead are tested for the four main gases that are emitted from every car’s exhaust. This is where the probe comes into play as mentioned above.

As for the four gases, we’ve talked about them in detail below…

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:

• Lambda
• Carbon monoxide
• Carbon dioxide
• Hydrocarbons

Let’s start with Lambda. A funny Greek word that seems very outdated, the ‘Lambda’ reading makes sure that your car is burning its fuel correctly. The technician will check the stoichiometric level to make sure you’re getting a nice, clean burn rather than one with too much air (a lean burn) or too much fuel (a rich burn).

Hydrocarbons are measured similarly to Lambda. Ideally, a car engine will burn all of the hydrogen and all of the carbon. If your car isn’t, it’ll fail this test because your car won’t be burning fuel correctly. This is sometimes known as a ‘dirty burn.’

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. Your catalytic converter plays a huge role in limiting the emission of that.

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

Which Cars Fail MOT Emissions Tests Most Often?

Petrol Or Diesel?

Diesel cars are much more likely to fail the MOT emissions test, with petrol cars regularly passing it with ease.

Older or Newer?

This might not be too surprising, but it’s older cars that fail MOT emissions test most often. This is usually because the category they fall into (Euro Cat 4, as opposed to Euro Cat 6) is simply worse at filtering out harmful emissions.

Older vehicles don’t use the same technology that modern cars have to filter and reduce the emissions that are kicked out into the air. For example, catalytic converters have advanced – early models would have been rudimentary compared to today’s.

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.

Can Emissions Test Failures Be Fixed During An MOT?

Not exactly. When an MOT test is being conducted, no repairs can be performed – the test must be completed from start to finish. Since it’s purely an inspection, the only thing that can be changed is the car’s headlight beam setter.

But, if a very minor problem requiring a quick fix is spotted and causes an MOT fail, it can be fixed whilst the car is still on the testing ramp. This is a classed as a PRS – Pass, Rectified at Station. This is most common towards the end of the working day, when the technicians might not have another car lined up to be tested after yours.

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!

On the other hand, if a good quality additive does the job, you might only have the pay out £30 to £40!

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

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