The conventional wisdom is that diesel cars fail their MOTs more than petrol cars. Emissions, DPFs, EGR valves — the narrative runs that diesel’s complexity punishes owners at the bay. The 2024 UK MOT data tells a more nuanced story.
The scale of the data
The 2024 UK MOT record covers roughly 32.6 million petrol tests and 28.6 million diesel tests — over 61 million combined data points across the two dominant fuel types. No other country publishes test-level data at this resolution. The sample is large enough to treat seriously.
The petrol fleet remains larger by test volume, which reflects the decade-long shift away from diesel post-2017. Diesel registrations peaked around 2016–2017 and fell sharply as manufacturers faced the post-emission-scandal regulatory environment. The result: the diesel fleet on UK roads today skews slightly older than petrol, but the gap is closing as late-2010s diesel cars move into mass-market ownership and their first MOT cycles.
Why the gap has narrowed
Two structural forces explain the compression.
The first is fleet renewal on the diesel side. The worst-performing diesel cohort — high-mileage urban cars where DPF blockage and EGR fouling peaked — has thinned. Cars that were being tested 8–10 years ago with clogged DPFs from city driving patterns have either been repaired or scrapped. The diesel fleet that remains skews toward cars owned by higher-mileage drivers, which is exactly the use case diesel engines were designed for. Motorway-heavy diesels have cleaner DPFs than urban diesels, and that distinction is now more visible in the aggregate numbers.
The second force is petrol’s own fleet problem. As the petrol fleet ages — driven by the sheer volume of 2010–2016 petrol registrations now hitting 10+ years — its aggregate pass rate is being diluted by older, higher-mileage cars carrying the same mechanical wear that historically characterised the diesel fleet. Suspension, tyres, and brake components don’t care what fuel the engine burns. A 2012 petrol Focus and a 2012 diesel Focus fail on the same items once the mileage accumulates.
What diesels are failing on now
The failure profile of the modern diesel MOT is not the emissions horror story it was five years ago. The top reasons for diesel rejection in 2024 are structural and mechanical, not exhaust-specific.
Tyres lead. Brake components — pads, discs, and linings — sit second. Suspension wear follows, with steering components alongside it. Lighting failures round out the top five.
That is, almost exactly, the same failure hierarchy as petrol. The diesel-specific items — DPF condition, exhaust smoke levels, EGR operation — still appear in failure data, but they’re no longer dominating the way they did. Owners have adapted: DPF regeneration drives are now well-understood, and the cars that remain on the road are more likely to be owned by people who understand what a diesel needs.
The petrol fleet’s silent ageing problem
Petrol’s pass-rate lead is real but overstated in public perception, partly because the comparison is often drawn between new petrol and old diesel rather than like-for-like age cohorts.
When you compare petrol and diesel cars registered in the same year band, the gap narrows further. A 2014 petrol car and a 2014 diesel car, tested in 2024, are failing on broadly similar items. The petrol won’t have DPF issues. The diesel won’t have the same catalytic converter wear patterns. But both will have aged suspension bushes, tyres past their prime, and lighting that hasn’t been maintained.
The fleet-level data masks this because petrol’s newer cohort — post-2018 petrol registrations are more numerous than post-2018 diesels — pulls the average up. Strip that out, and the story is less dramatic.
Diesel’s structural advantage in one area
One area where diesel genuinely outperforms at the MOT: brake wear. Diesel cars tend to be heavier, but they’re also more frequently fitted with stronger braking systems and driven by owners who do higher annual mileages — which paradoxically keeps brake pads in better condition. Pads that are used stay bedded; pads that sit rarely used can corrode at the edges and fail on minimum thickness even at low mileage.
This is visible in the data. High-mileage diesel models show proportionally fewer brake-related MOT failures than their mileage would suggest. The same cars driven on short urban trips would look very different.
What changes if you’re buying used
The fuel-type framing matters less than it used to when making a used-car purchase. The practical questions are:
What was the car used for? A diesel driven 15,000 miles a year on motorways has a healthier DPF than a diesel driven 7,000 miles a year in a city. The use pattern matters more than the badge.
What’s the age-cohort pass rate for this specific model? Fleet-level petrol vs diesel comparisons smooth out enormous variation between models and year bands. The MOTCost data lets you look at the car you’re actually buying, not a category average.
Has the car had its DPF or emissions-related work done? A diesel with a documented DPF clean or EGR service is better positioned than one with no record and unknown history.
The pass-rate gap between petrol and diesel is now narrow enough that it should not be the deciding factor in a used car purchase. Age, mileage, use pattern, and service history carry more predictive weight than fuel type alone.
The longer trend
The diesel-vs-petrol framing will become less relevant as hybrid and electric vehicles take a larger share of MOT tests. In 2024 their combined volume is still small relative to 61 million petrol and diesel tests. But the trend is set. Within a decade, the comparison table will have a third significant column, and the diesel-specific concerns that drove the narrative through the 2010s will be a smaller part of the overall picture.
For now, the data is clear: the gap is real, it favours petrol, and it is narrower than the conventional wisdom suggests.