Methodology

The MOT Risk Index compares each model's MOT failures with what is normal for cars of the same age.

means normal for the age: below 100 fails less often, above 100 more often: 100means normal for the age: below 100 fails less often, above 100 more often

First-attempt failures divided by those expected at the same ages, multiplied by 100

Method: DVSA MOT bulk data · release dvsa-2024-v2 · calendar 2024

In plain English: we take every official MOT test from 2024 (52 million of them), count how often each car family failed at its first attempt, and then ask one fair question: is that more or less than you'd expect, given how old those cars are? Old cars fail more — that's normal. Our index is 100 when a family fails exactly as much as its age predicts; 118 means 18% more than expected; 76 means 24% less. Everything below explains the exact maths behind that one idea.

This site computes vehicle-family statistics from the DVSA "Anonymised MOT tests and results" bulk dataset (Open Government Licence v3.0). Current data release: dvsa-2024-v2— calendar-year 2024, 66,857,355 test records. Every published figure carries a claim ID that maps to a specific script in our versioned build pipeline; nothing is hand-edited and no generative model writes or alters any number on this site.

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Full method and limitations

1. What counts as a test, and what counts as a failure

We use initial tests only: DVSA test type NT (“Normal Test” in the DVSA lookup); retests RT (“Re-Test”), PL and PV (partial retests) are excluded. Class-4 vehicles (cars), outcomes P (pass), F (fail) and PRS. PRS means the vehicle did not pass its initial examination: a failing defect was found and rectified at the test station within an hour. Our headline initial-failure rate therefore counts F + PRS — the share of cars that arrived at the test with at least one failing defect. The stricter "walked away with a fail certificate" rate (F only) is shown alongside. Retests are excluded from all headline rates and analysed separately.

2. The index: age-adjusted MOT initial-failure ratio

Raw failure rates punish models with older fleets. The index corrects for this by comparing a family's observed initial failures with the number expected for its exact age profile:

Index = 100 × observed initial failures ÷ Σ (tests in age stratum × national stratum rate)

Age strata: under-3, single years 3–19, 20-plus, unknown. No clamping of extreme ages.
The reference population is all class-4 initial tests in the same window (2024), frozen per data release. When a new release is published, previous baselines are preserved — time-comparisons are only made against frozen baselines.
100 = exactly the failure level expected for that fleet's age profile. Below 100 = fewer failures than age predicts; above = more.
Confidence intervals are computed on the index itself (binomial interval on the observed count scaled by the frozen expected value). Where the 95% CI includes 100, the verdict is labelled directional and no colour rating is assigned.

The index is a test-outcome statistic, not a general "reliability" score: the MOT does not examine engine internals, gearboxes, air conditioning or infotainment. Residual confounders not yet adjusted for: annual mileage, fleet/private use mix, fuel mix within a family. Mileage stratification is planned; until then, per-fuel splits are shown wherever the spread between powertrains exceeds 5 percentage points.

3. National baseline (release dvsa-2024-v2)

Age	Initial-failure rate
3	12.4%
4	13.1%
5	15.1%
6	17.5%
7	20.2%
8	23.3%
9	26.5%
10	30.1%
11	33.2%
12	35.7%
13	38.2%
14	40.4%
15	41.6%
16	43.1%
17	43.1%
18	43.9%
19	43.5%
20+	41.0%

4. Grain: what a "family" is

Pages aggregate at model-family level (e.g. all Fiesta variants), built by an explicit published mapping from DVSA's free-text make/model fields. We do not split by generation yet — the word "generation" is deliberately absent from data pages until a validated generation-resolver exists. Where powertrains within a family diverge materially, a per-fuel table is shown.

5. Suppression and small samples

Every figure is printed with the number of tests behind it. A model page states its confidence in its first sentence: under 1,000 initial tests it reads, for example, "a rough estimate from 268 first tests" (127 of the 752 models), and 1,000 to 5,000 tests reads "based on 3,854 first tests".

Rough estimates. Inside a page, a figure is published from a stated minimum instead of being hidden: 30 tests for a single year of age, 40 for a median mileage (80 for its quartiles), 40 cars for a next-failure age band, 50 tests for an odometer band within an age group, 100 tests for a postcode area. Between that minimum and 500 tests the row says "rough estimate" and gives its 95% interval, so you can see how wide it is. No percentage is published from fewer than 30 tests.

When we name a best or a worst. Words such as best, worst, most, steepest or "more often than chance" are used only when the 95% interval rules the difference in and the difference is large enough to matter: at least 5 index points between two models or build years, at least 3 percentage points for a postcode area against the model's own rate, at least 5% above expectation for a brand's failure group. A comparison names a failure group where two models diverge only when the 95% interval of the gap excludes zero and the gap is at least 0.5 items per 100 tests and at least 5% of the higher expected rate. Higher mileage is said to go with more failures only when, inside the same age group, the interval of the highest odometer band lies wholly above that of the lowest. Otherwise the page says that nothing stands out.

Outside sources. Other countries' inspections are matched by make and model name; a Vauxhall is matched to the Opel of the same name and the page says so ("as the Opel Corsa"). US complaint and service-bulletin counts appear only for an explicit pairing of the same vehicle within stated US model years (for example the Honda Jazz as the US Fit, model years 2015–2020); they are counts, not a UK failure rate.

6. Known caveats of the source data

Defect severity fields (dangerous/major/minor) are absent from the 2018–2021 releases; severity analysis uses releases where present.
The DVSA 2024 files repeat many test records: each monthly file is several export chunks joined together, and the same test appears two to eight times (66,857,355 rows hold 42,598,624 distinct test IDs). From release dvsa-2024-v2 we keep one record per test ID. Where the copies disagree about the vehicle itself — make, model, test class or vehicle ID — the test is excluded (107,659 tests, 0.25%); where they disagree only about the first-use date, the car's age is treated as unknown. Identical defect entries within one test are counted once, which can undercount genuinely repeated entries by up to about 0.4% (their share in the 2019–2023 files, which have no repeated tests). Release dvsa-2024-v1 counted the repeats; see the changelog.
Unknown first-use dates are encoded by DVSA as 01/01/1971 — we treat these as "unknown" and exclude them from age curves (196 of 33,012,215 class-4 initial tests in 2024; a further 427,684 have no usable first-use date, mostly because their repeated copies disagreed). The issue is more material in older releases.
Hybrid fuel coding was corrected by DVSA only from the 2022 dataset onward.
Make/model are free-text fields with typos and trim tails; our mapping is conservative and published.
DVSA's own compliance survey (2021–22, repeated 2023–24) estimated that roughly 1 in 10 vehicles receiving a pass should have failed — and that errors run in both directions. This bounds the precision of any MOT-derived statistic, ours included.

Bounds of applicability

These statistics describe MOT test outcomes of vehicle populations in Great Britain, calendar 2024. They are not: predictions for any individual vehicle; measures of breakdown risk (a different construct — see ADAC-style data); or comparable across countries without harmonisation. Review date for this release's claims: 2026-10-30 (quarterly).

Definitions used on every page

MOT pass rate: The share of initial MOT tests that ended in a pass, counting a defect fixed during the same test (PRS) as a pass. It is 100 minus the strict failure rate.
Passed first time: The share of normal tests (DVSA test type NT) with result P: no failing defect was found. PRS counts as a failure here, and advisories and minor defects can still be present. It is 100 minus the initial-failure rate.
PRS: Pass after rectification at station: the car failed, the fault was fixed at the test centre within an hour, and the test was recorded as a pass. DVSA's own wording: “Pass after Rectification at Station - The process where defects may be rectified within one hour after the test, but before recording the results on the MOT Testing Service” (MOT testing guide: abbreviations and definitions).
Advisory: An optional note about an item that is not bad enough to fail. DVSA's inspection manual: “When an item is not sufficiently deteriorated to justify rejection, there may be an option to select ‘advisory’ to inform the presenter of this fact.” A minor defect is different: “Unlike advisory items, the use of minor defects, where appropriate, is mandatory”, and a car with only minor defects still passes (MOT inspection manual: introduction). Advisories are counted per 100 tests.
Dangerous defect: DVSA's inspection manual: “dangerous - defects that are a direct and immediate risk to road safety or having an impact on the environment”; “If a vehicle has any major or dangerous defects, it must be failed and a refusal notice issued” (MOT inspection manual: introduction). GOV.UK adds that you can take the vehicle away only if the current MOT is still valid and “no ‘dangerous’ problems were listed in the MOT” (MOT test result). Counted per 100 initial tests.
MOT Risk Index: Observed first-time failures divided by the failures expected for the same age profile, times 100. 100 is normal for the age; the index allows for age, not mileage. The 95% interval shows its uncertainty.

How to cite

MOT Risk Index (2026). UK MOT test statistics 2024, data release dvsa-2024-v2; full dataset motriskindex-families-2024-v5, DOI 10.5281/zenodo.22975051. Each page shows its own citation with its address.

7. Reproducibility

Each page shows its dataset_release_id. The full computation chain — cleaning rules (embedded header rows, quote escaping), family mapping, index formula, CI method — is versioned in git and pinned per release. A claim registry mapping every published figure class to its script ships with each release.