# What London cars fail on

> Brakes, suspension, tyres, lamps and every other recorded reason class 4 vehicles fail an initial MOT.

- Category: Transport
- Cadence: annual
- Status: Published on a annual cadence.
- Dataset key: `mot-failure-systems`
- Source: [DVSA](https://open.data.dvsa.gov.uk/mot-anonymised/index.html)
- Licence: Open Government Licence v3.0 — [terms](https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/)
- Canonical page: [https://londongraph.com/g/what-london-cars-fail-on](https://londongraph.com/g/what-london-cars-fail-on)
- Machine-readable record: [https://londongraph.com/data/what-london-cars-fail-on](https://londongraph.com/data/what-london-cars-fail-on)

## Methodology

DVSA's separate failure-item archive joined by test ID to class 4 initial tests for vehicles kept in the eight London postal areas. Retests and the twelve boundary-straddling postcode areas are excluded. Counts are REASON-FOR-REJECTION RECORDS, not vehicles: one test can contain several defects, minor items and advisories, so records cannot be added and described as the number of cars failing. The main ranking includes failed items only; the severity view separately retains advisory, minor, major and dangerous inspection records. Current DVSA item-detail and hierarchy lookups assign each record to its published inspection system. Changes to the MOT regime in 2018 limit long-run comparison.

## Visualisations

- [Failure systems](https://londongraph.com/g/what-london-cars-fail-on): Rank published inspection systems by recorded failure items in each annual extract.
- [Severity](https://londongraph.com/g/what-london-cars-fail-on?view=severity): Keep advisory, minor, major and dangerous inspection-item records visibly separate.
- [System × severity](https://londongraph.com/g/what-london-cars-fail-on?view=system-severity): Cross each inspection system with severity in the latest joined release. Caveat: Cells count inspection-item records, not unique vehicles.
- [Detailed reasons](https://londongraph.com/g/what-london-cars-fail-on?view=details): Use a treemap for the forty most frequently recorded detailed failure reasons. Caveat: A vehicle can contribute several failure reasons during one test.

## Citation guidance

Attribute the original publisher named above and identify LondonGraph as the visualisation and methodology layer. Do not describe live values as a historical census, and do not present placeholder values as observations.