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.
A car that’s pumping too much pollution into the air can be costly to repair, but not impossible. Here’s what needs to be done, and what alternative options you have, including selling it as scrap without a valid MOT.
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 Happens Next?
The next course of action after any MOT fail should be consider whether it’s worth fixing the car. If the repair costs outweigh what you think is worth paying for the car, then consider scrapping or salvaging it instead.
If you want to fix the car up, that should be your immediate next step – ask your MOT technician if they can do the work, or think about going elsewhere if you know where you can get a better deal. Just be wary of the requirements around partial retests.
How Much Does It Cost To Repair An Emissions Fail Car?
The price you’ll pay to get your vehicle fixed and MOT retest-ready can vary depending on what actually needs doing to it.
If you’re looking at a relatively simple fix, like new sensors or cleaning air filters, expect a bill of around £100. However, something like a replacement catalytic converter – a notoriously expensive part – can set you back about £1,000!
Why Do Cars Fail Their MOT Emissions Tests?
Cars fail their emissions tests when the measured readings of the fumes their car puffs out are above a legal minimum.
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.
Want a complete run-down of everything that can fail an MOT? We’ve got just that right here.
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
How Do I Get My Car To Pass The Emissions Test?
Obviously, the best thing you can do is ask a mechanic to perform whatever work is necessary, whatever the cost. However, you don’t all have the luxury of that kind of financial freedom. So, what else can you do?
Some of your options are actions you can take whilst driving, whereas others will benefit the performance during the test itself:
- 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.
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!
If your catalytic converter has become worn out, or worse, has been stolen, find out how you can still make the most money from selling your car.
