How to read MOT pass rates without being misled
By Plateworthy · Updated · Editorial policy
Two accurate MOT percentages can tell different stories. Before a pass rate changes your shortlist, identify what was counted, which cars were included and how old they were at the test.
A worked example: one denominator, two questions
Imagine 1,000 normal initial tests: 800 pass, 150 fail, and 50 pass after rectification at the station (PRS). These are illustrative figures, not a study of a real model.
The final recorded pass rate is (800 + 50) ÷ 1,000 = 85%. The initial failure rate is (150 + 50) ÷ 1,000 = 20%. They add to 105% because the 50 PRS tests were initially defective and ultimately passed. A headline that does not explain PRS can hide this distinction.
A failure percentage needs its denominator
Suppose tyres were a failure reason on 80 of those tests. That is 8% of all tests, or 40% of the 200 initial failures. Both statements can be correct; they answer different questions.
One test can fail on several components. Count each test once per category rather than counting every repeated tyre or lamp defect. A failure-category table is not a list of independent probabilities you can add together.
Age, fuel and survival change the comparison
A model with a large elderly fleet is not directly comparable with a recently launched model whose oldest cars have only just reached MOT age. Diesel, petrol, electric and hybrid cohorts may also reflect different usage patterns.
Use matched age and fuel groups when there are enough observations. Even then, mileage, maintenance, which cars survive to test age and how the model names were grouped can affect results. Our cohorts are descriptive; we do not label them a validated reliability score.
Tests are not the same as vehicles
A vehicle can be tested in successive years, so 50,000 test records need not mean 50,000 different cars. Retests can also inflate a denominator if they are mixed with initial tests. A reproducible analysis should identify the unit and remove overlapping copies of the same test ID.
Our lifetime annualised mileage metric is odometer divided by age at test, averaged across usable observations. It is not the miles driven in the preceding year. Its valid-reading sample can be smaller than the pass-rate sample.
Turn a pattern into a question, not a verdict
If a category is frequent for the relevant cohort, ask to see the individual car's MOT entries and repair invoices for that area. An old advisory that disappears does not prove a repair was completed; a test can describe only what was observed at that inspection.
Use a history report for the risks it actually covers, inspect the vehicle and obtain specialist help where needed. Aggregate MOT outcomes do not cover finance, theft, every accident, servicing or battery capacity.
Common questions
- Is a high MOT pass rate proof a car is reliable?
- No. It describes test outcomes in a particular sample, affected by age, use, preparation and the test rules. It is not an assessment of the car you are viewing.
- Why are some Plateworthy values marked legacy?
- Those older aggregates did not retain source periods and complete calculation evidence. We label them, withhold unverified mileage/failure rankings and do not apply the new methodology retrospectively.
Sources and further checks
Primary guidance checked for this update. Rules and individual circumstances can change; use the linked official service or advice where a decision depends on it.