Exhaust MOT Failures in the UK: What 20 Years of DVSA Data Show (2005-2024)

Exhaust problems are one of the familiar reasons UK cars fail their MOT – but how common are they really, and is the picture changing? Based on DVSA MOT records spanning 2005-2024, exhaust-related MOT items have produced more than 25.8 million failure items across two decades of testing. Yet the rate at which cars fail on the exhaust system has fallen by 75% since 2006. This guide sets out what the data show about exhaust MOT failures: which MOT items they are recorded under, how often each one comes up, how the rate has changed over twenty years, and – just as importantly – what the records cannot tell you.

How common are exhaust MOT failures across the UK?

According to DVSA MOT records from 2005-2024, a total of 15,077,554 "Exhaust system fault" failure items were recorded, alongside 20,379,335 exhaust system advisories. In 2024 alone, 391,858 exhaust system failure items were logged across 34,233,620 UK MOT tests – a rate of 1.14 per 100 tests.

To put that in perspective, in 2006 the exhaust system failure rate stood at 4.55 per 100 tests. By 2024 it had fallen to 1.14 – a 75% drop in the rate. Exhaust problems still turn up at hundreds of thousands of MOT tests every year, but the rate per test is now roughly a quarter of what it was in 2006.

What exhaust problems cause MOT failures?

The DVSA records exhaust-related failures under several separate MOT items. Across all of them, 25,872,815 failure items were recorded between 2005 and 2024. Not every item covers the whole period, so the years each one appears in the records are shown below.

• Exhaust system fault (recorded 2005-2024): 15,077,554 failures – 58.3% of exhaust-related failures – and 20,379,335 advisories
• Catalytic converter efficiency low (recorded 2005-2018): 10,275,806 failures – 39.7% – and 114,412 advisories
• Exhaust emissions above threshold (non-catalyst) (recorded 2005-2018): 451,438 failures – 1.7% – and 17,931 advisories
• Exhaust leak (recorded 2018-2024): 56,140 failures – 0.2% – and 197,227 advisories
• Exhaust heat shield or sound insulation damaged (recorded 2018-2024): 11,877 failures – less than 0.1% – and 22,404 advisories

The general "Exhaust system fault" item dominates, at 58.3% of all exhaust-related failures and the only one of these items recorded across all twenty years. The catalytic converter item is the second largest at 39.7%, even though it appears in the records only from 2005 to 2018. Together, the catalytic converter and non-catalyst emissions items account for 41.5% of exhaust-related failures.

Catalytic converter and emissions failures

The "Catalytic converter efficiency low" item was recorded from 2005 to 2018, producing 10,275,806 failures and 114,412 advisories during that time. The separate "Exhaust emissions above threshold (non-catalyst)" item, recorded over the same years, added 451,438 failures and 17,931 advisories.

It is worth being clear about what these figures are. DVSA MOT records capture that an item failed, not why. The data cannot tell us what lay behind any individual catalytic converter or emissions failure – only that the car did not meet the standard at the time of its test.

Exhaust leaks and heat shields

From 2018, exhaust leaks appear in the records as their own item. Between 2018 and 2024, 56,140 exhaust leak failures were recorded, along with 197,227 advisories.

Damaged exhaust heat shields or sound insulation, also recorded from 2018, produced 11,877 failures and 22,404 advisories over the same period. Both items are small next to the general exhaust system fault – but for both, advisories outnumber failures.

How has the exhaust failure rate changed since 2006?

The fall in the exhaust system failure rate is one of the clearest long-term trends in UK MOT data. The rate actually rose from 4.55 per 100 tests in 2006 to 4.77 in 2007 – its highest point in the records – and has fallen in every year since:

• 2007: 4.77 exhaust system failures per 100 MOT tests
• 2012: 3.31 per 100 tests
• 2018: 1.90 per 100 tests
• 2024: 1.14 per 100 tests

Because this is a rate per 100 tests, it already allows for the growth in the number of MOT tests, which rose from 24.3 million in 2006 to 34.2 million in 2024. The number of exhaust system failures fell too, from 1,103,817 in 2006 to 391,858 in 2024.

What the MOT data cannot do is explain the fall. The records show that fewer exhaust system failures were recorded per test; they do not record the age, mileage, parts or history of the cars tested, so they cannot say why.

How do advisories fit into the exhaust MOT picture?

Alongside the 15 million exhaust system failures recorded from 2005-2024, the DVSA data show more than 20.3 million exhaust system advisories over the same period. An advisory is a note on a passing MOT certificate: the item met the minimum standard on the day, but the tester recorded something worth keeping an eye on.

For exhaust leaks, advisories outnumbered failures by more than three to one between 2018 and 2024 – 197,227 advisories against 56,140 failures. Our look at the MOT advisories drivers most often ignore covers advisories across the whole car.

An advisory is not a failure, and a car carrying one has still passed its MOT. But it is the tester telling you where to look, so it is sensible to have an exhaust advisory checked by a garage rather than waiting for the next test.

What the data cannot tell us about exhaust failures

DVSA MOT records give a very large view of what failed and when, but they have important limits. They record that a particular item failed an MOT, not why it failed. For a catalytic converter efficiency failure, for example, the data do not say what caused it.

The figures in this guide also say nothing about component age, mileage, or how a car was driven. We cannot tell from these records whether a failed exhaust was on a three-year-old car or a fifteen-year-old one, or whether it had covered 30,000 miles or 130,000. Any claim about typical exhaust lifespan or mileage at failure would need data beyond what MOT testing records.

Coverage also differs by item. Catalytic converter efficiency and non-catalyst emissions appear from 2005 to 2018, while exhaust leak and heat shield items appear only from 2018 onward. We note these date ranges where relevant but do not speculate on why the recording changed – the data simply show the years each item was captured. For MOT failure data across every category, our DVSA fault data hub breaks it down by make, model and problem.

Preparing your car's exhaust for its MOT

With the exhaust system fault still among the MOT items recorded hundreds of thousands of times a year, it makes sense to have your exhaust looked at before the test if anything seems wrong. Obvious things to notice are a change in how the exhaust sounds, anything hanging lower than it should, or visible damage.

If your car received an exhaust-related advisory at its last MOT, it is worth having that item checked before the next test rather than on the day. Local car repair garages can inspect the exhaust and tell you whether anything needs doing, which gives you time to deal with it on your own terms instead of after a failed test. What any repair involves will depend on your car and the garage, so it is worth getting a quote for your own vehicle. You can also set a free MOT reminder so the test date doesn't catch you out.

Frequently Asked Questions

What is the most common exhaust problem at MOT?

The "Exhaust system fault" item is the most common, accounting for 58.3% of all exhaust-related MOT failures recorded from 2005-2024 – 15,077,554 failure items in total. It is also the only exhaust item recorded across all twenty years.

How many cars fail their MOT on the exhaust each year?

In 2024, 391,858 exhaust system failure items were recorded across 34,233,620 UK MOT tests, a rate of 1.14 per 100 tests. That rate has fallen by 75% since 2006, when it stood at 4.55 per 100 tests.

Can I pass an MOT with an exhaust advisory?

Yes. An advisory is recorded on a passing MOT – the item met the standard on the day, but the tester noted something worth watching. It is still worth having an exhaust advisory checked by a garage before your next test.

Are catalytic converter failures recorded separately in MOT data?

The "Catalytic converter efficiency low" item appears in DVSA MOT records from 2005 to 2018, with 10,275,806 failures over that period – 39.7% of all exhaust-related failures. It does not appear under that item after 2018.

Conclusion

Twenty years of DVSA MOT records show a clear trend: the exhaust system failure rate has fallen by 75% since 2006, from 4.55 per 100 tests to 1.14 in 2024, and it has fallen every year since its 2007 peak. Exhaust problems are still common in absolute terms – nearly 392,000 exhaust system failure items in 2024 alone – and the general exhaust system fault and catalytic converter efficiency items together make up 98% of exhaust-related failures recorded. If your MOT is coming up and you're concerned about your exhaust, you can get quotes from garages near you for your own car. For a broader look at which cars and components fail most often, explore our DVSA fault data or look up exhaust problems on UK cars by make and model.