# Reliability data — how AutoScout scores UK used cars

Live version: https://autoscout.fyi/guides/reliability-data

AutoScout's reliability scores come from 33 million DVSA MOT tests — the complete UK MOT dataset from 2005 onward, released annually by the Department for Transport. For every model and generation we analyse:

- **First-test pass rate**: how often a car of that model, at its first MOT, passes without advisories. A strong proxy for build quality in the first three years of ownership.
- **Three-year-test pass rate**: how the car holds up once out of warranty. This is where mass-market cars separate from premium ones.
- **Fail reasons**: the most common reasons tests are failed, normalised by mileage band. Reveals the expensive faults that tend to arrive at 50,000–80,000 miles.
- **Advisory rate**: even a passed MOT can flag advisories (brakes, tyres, corrosion, suspension). We count these because they predict the next year's repair bill.

## What "Good / Fair / Poor" means

- **Good** — first-test pass rate above the segment average, three-year pass rate still competitive, no clustered fail reasons above the expected baseline.
- **Fair** — roughly in line with the segment average. Reliable enough for most buyers, but shop on service history and condition.
- **Poor** — measurably below the segment average, usually because of one or two recurring faults (wet belt, DPF blockage, EGR failure, turbo, electrics). We flag the specific fault on each car's page.

## Why MOT data beats surveys

Owner-satisfaction surveys (JD Power, Which?) rely on self-reported memory and small samples — a few thousand responses per brand. The MOT dataset is the entire UK fleet: around 40 million annual tests, with binary pass/fail and a free-text failure reason for every failed item. That makes it resistant to recall bias and skew from enthusiast owners.

## Combining reliability with depreciation

A cheap unreliable car often costs more over three years than a reliable one a few thousand pounds dearer. Each car report combines the reliability score with a depreciation forecast and typical repair costs for the common faults, producing a single "total cost to own" view.

Data sources: DVSA MOT history (gov.uk), AutoTrader UK pricing, CAP / Glass's Guide depreciation logic, manufacturer recall data.