Tyres  /  Tyre News /  Help and Advice  / MOT Car Emissions Failure: What to Do If Your Car Fails

Amongst the several defects that can fail your MOT, one of the more complicated faults that can fail your car’s test is its emission levels.

Emission levels are a hard defect to gauge for your own vehicle, but can be checked on the GOV.UK website.

However, there are various factors that can lead to elevated emissions exceeding MOT limits. As a result, your car may fail its MOT test if these issues aren't addressed.

This article will cover what an emissions-related MOT failure means, common causes behind it, how to fix the problem, and what steps to take next to get your vehicle road-legal again.
 

What Is an Emissions test in an MOT?

When your car goes in for its MOT, one of the key checks it must pass is the emissions test.

This part of the test ensures your vehicle isn’t releasing excessive harmful gases into the environment – and it’s particularly strict when it comes to cars with older or poorly maintained engines.

The test checks your car's exhaust emissions while idling and under acceleration. For petrol vehicles, these checks focus on three main areas:

• Carbon Monoxide (CO): Must be ≤0.2% at fast idle and ≤0.3% at normal idle
• Hydrocarbons (HC): Must be ≤200 parts per million (ppm) at fast idle
• Lambda reading: Should sit between 0.97 and 1.03 to indicate a balanced fuel-air mix

For diesel vehicles, there’s no CO or HC measurement – instead, the tester checks for visible exhaust smoke.

Any smoke, particularly dense blue or black coloured smoke, will immediately fail the MOT test.

The condition of the Diesel Particulate Filter (DPF) is also crucial –  if it’s missing or visibly damaged, the car will automatically fail.

So, while the emissions test might seem like just another MOT checkbox, it plays a vital role in keeping our roads – and the air we breathe – clean.
 

Petrol vs Diesel: Emission Test Standards Explained

Petrol and diesel vehicles go through the same emissions testing process in principle, but the way they’re assessed differs significantly.

Petrol cars are tested using a gas analyser, with strict limits set on CO, HC, and the Lambda reading. These measurements give an insight into how efficiently the engine is burning fuel and how cleanly it’s running.

Diesel vehicles, however, are subject to a smoke opacity test. Rather than analysing gases, testers look for visible signs of pollution from the tailpipe. Any smoke seen during revving is a red flag – and if the Diesel Particulate Filter (DPF) is damaged or removed, that’s an automatic MOT fail.

Knowing which category your car falls into helps you understand what’s being tested and why – especially if you’re troubleshooting a recent fail.
 

Why Did My Car Fail the Emissions Test?

There’s no single reason why a car might fail its emissions test – but a handful of common faults are usually to blame.

• Faulty sensors – A failing oxygen sensor or mass airflow sensor can disrupt your car’s fuel-to-air mix
• Clogged or damaged DPF (for diesels) – If your diesel particulate filter is blocked or missing, emissions will spike
• Worn spark plugs or ignition problems – Poor combustion can result in high HC or CO levels
• Dirty air filters – Restricted airflow can upset the engine’s balance
• Engine running rich – If your car is using more fuel than it should, CO readings will rise
• Check engine light – A lit warning light is often linked to emissions-related faults

If your car has failed its MOT on emissions, chances are one or more of these issues is at play.
 

How to Check Your Car’s Emission Levels

While emissions faults aren’t always easy to spot, there are a few ways to check if your car might be at risk of failing.
Start by using the GOV.UK MOT history tool. Enter your reg number, and you’ll be able to see if your car has previously failed on emissions – along with any advisories.

You can also look out for tell-tale signs, such as:

• Black or blue smoke from the exhaust
• Reduced fuel economy
• Rough idling or engine hesitation
• A lit engine management light

If you’re in doubt, many garages also offer pre-MOT emissions checks or diagnostics – giving you peace of mind before the test.
 

Can You Drive a Car That Failed the MOT on Emissions?

In short: not legally, unless you’re driving it to a pre-booked MOT retest or repair appointment.

If your MOT has expired – or your vehicle failed its MOT on emissions or any other major defect – it’s illegal to drive it on public roads (except for the specific journeys mentioned above). Doing so could land you with a £1,000 fine and possibly invalidate your insurance.

So, if your car’s failed the emissions test, it’s best to get it repaired quickly and safely booked back in for a retest.
 

How to Fix Emission Test Failures

The good news is that most emissions-related MOT failures can be fixed – and often without major expense.

Here’s what might be needed, depending on the root cause:

• Oxygen or lambda sensor replacement
• DPF regeneration or cleaning (for diesels)
• Air filter or spark plug replacement
• Fuel system cleaning or engine flush
• Software updates or fault code clearing

Some motorists also use fuel additives or take the car on a longer drive before the MOT to help the system run cleaner – though this is more of a short-term trick than a long-term fix.

A proper diagnosis from a garage will help pinpoint the issue and get it sorted swiftly.
 

How to Prevent MOT Emission Failures in the Future

Keeping your car’s emissions in check isn’t just about passing the MOT – it’s also about efficiency, performance, and the environment.

Here’s how to stay ahead:

• Service your car regularly – Fresh oil, filters, and spark plugs all help
• Avoid short journeys – Longer drives help clear out the exhaust system, especially for diesels
• Use high-quality fuel – This can improve combustion and reduce deposits
• Keep an eye on the dashboard – Never ignore the engine warning light
• Warm up the engine before testing – A hot engine tends to perform better in emissions tests

By following these tips, you’ll not only boost your chances of passing the MOT first time – you’ll also help your car run cleaner and more efficiently year-round.