How our MOT reliability score works

Every vehicle we check gets a reliability score between 5 and 100. That number is our own work, not something published by the DVSA or DVLA. Because a score is only useful if you understand what went into it, this page documents the entire calculation — every weighting, every penalty, and the places where it can be wrong.

Why a Score At All?

Raw MOT history is genuinely hard to read. A car with fourteen tests, thirty advisories and two failures might be a well-maintained vehicle with an honest owner, or a tired one that keeps scraping through. Reading that properly means holding several things in your head at once: how often it failed, how serious the failures were, whether things are getting better or worse, and whether the mileage makes sense.

The score is an attempt to do that arithmetic consistently, so that two cars can be compared on the same basis. It is a starting point for a conversation with the seller — not a verdict.

Score Composition at a Glance

The chart below shows the maximum each component can contribute or subtract.

The final result is clamped between 5 and 100, so no single penalty can drive a score to zero.

The Calculation, Step by Step

When full DVSA test history is available, the score is built as follows.

What Changed in Version 2 (November 2026)

We rebuilt the score after testing it against real MOT histories and finding three weaknesses.

• Retests were counted as separate tests. A same-day fail and retest pass looked like a 50% year. MOTs are now grouped, so one problem costs once.
• Age was ignored. A 60% first-time pass rate is normal for a car over twelve years old and poor for a young one. The score now compares each car with a typical car of the same age.
• Advisories and rust didn't count. Repeated advisories and structural corrosion are among the best warnings of future trouble, so they now affect the score.

In our checks, cars with consistently good records for their age moved up by around five points, while cars that failed most of their MOTs moved down by ten to fifteen. Clean records still score 100. We also stopped deducting points from cars without MOT history when tax status is unknown.

Two Decisions That Make the Score Different

Most of the above is arithmetic anyone could reproduce. Two parts involve judgement, and they are the reason our score does not simply mirror the raw pass rate.

Not every major failure is a bad sign

The MOT scheme records a worn tyre and a corroded subframe under the same heading: major. Treating them identically would be misleading. A car that failed on a blown bulb is not comparable to one that failed on structural rot, but the official record gives you no way to tell them apart at a glance.

So we read the defect text and sort major failures into two groups. Items we treat as routine consumables include tyres, bulbs and lamps, wipers, washers, the horn, mirrors, number plates, reflectors and headlamp aim. These still appear in the defect list you see — we never hide them — but they do not drag the score down. Everything else is treated as a serious defect and does affect it.

This is a judgement call, and it is the part of the methodology most open to disagreement. A tyre failure genuinely is a safety issue. Our reasoning is that it says far more about the previous owner's servicing habits than about the fundamental condition of the vehicle, and that a buyer comparing cars cares most about the latter.

A mileage drop is not always clocking

The obvious way to detect odometer tampering is to look for a reading that goes backwards. Done naively, this produces a lot of false accusations, because some vehicles record in kilometres for part of their life and miles for the rest. Switching from kilometres to miles makes the number fall by roughly 38% with nothing untoward having happened.

Before flagging a decrease we therefore check whether the ratio between the two readings sits close to the kilometre-to-mile conversion factor of about 1.609. If it does, we treat it as a unit change rather than tampering and carry on. Where the API does not state the unit at all, we infer it from the shape of the sequence.

The trade-off is deliberate: a genuine rollback that happens to land near that ratio would be missed. We would rather occasionally miss one than routinely accuse honest sellers.

When There Is No MOT History

Some vehicles have no DVSA test history — typically because they are under three years old and have never needed an MOT, or because no MOT record is available yet. Scoring these the same way would be meaningless, so a separate and much simpler calculation applies.

It starts from a neutral baseline of 50. A valid MOT status adds 25 and an invalid one subtracts 20. Being taxed adds 10 and being untaxed or SORN subtracts 10; if tax status is unknown, it has no effect. Vehicles up to three years old gain 10, and those up to six years old gain 5.

This is a much weaker signal and should be read as little more than a status summary. Where you see a score derived this way, the underlying data simply does not support a reliability judgement.

What the Score Cannot Tell You

A number this compact necessarily throws information away. These are the limits we know about.

• It only sees MOT days. A car can break down repeatedly and still pass every test. Nothing about servicing, clutches, gearboxes, air conditioning or electrical gremlins appears in MOT data at all.
• It cannot see repairs. A failure followed by a proper repair and a failure followed by a bodge look identical in the record.
• The score itself doesn't know the model. It compares a car with every car of the same age, not with others of the same make and model. Some models fail more often for reasons of design, so check the model's own record on our MOT data by model pages.
• Low mileage is not scored as good. Very low mileage can indicate long periods standing, which brings its own problems.
• Recent cars score on thin evidence. A car with one test has no meaningful track record, and the history-depth bonus reflects that only weakly.
• Provenance data is not always available. Where it is missing, those penalties simply do not apply — a clean score is not proof of a clean title.

Treat a high score as a reason to keep looking at a car, and a low score as a reason to ask harder questions. Neither replaces a test drive, an inspection, or a paid history check.

Data Sources

Test results, defect text and odometer readings come from the DVSA MOT History API. Vehicle details and MOT status come with that record; live tax status from the DVLA Vehicle Enquiry Service is being added. The pass rates by age come from our analysis of the DVSA's anonymised MOT data for 2025. These are official UK government sources. The scoring, defect classification, unit-switch detection, comparison logic and written analysis on this site are our own.

We do not alter the underlying government data, and where our interpretation differs from the raw record you can always see the raw record alongside it.

Corrections

If you believe the score is wrong for a specific vehicle, or you disagree with how we classify a particular defect, we want to hear about it — that feedback is how the methodology improves. Please get in touch with the registration and what you think is wrong. If the underlying government data itself is incorrect, that must be corrected at source with the DVSA before it will change here.