The MOT Files

Known faults, from the test record

Seat Mii common problems

What fails on the Seat Mii at MOT, which parts are behind it, and how each model year compares. From 18,638 DVSA test records.

Data updated

Quick answer

The Seat Mii's most common MOT problems are tyres, catalytic converter and emissions, bulbs and lamps, recorded in 27.1%, 24.2% and 18.4% of failed tests respectively. By model year, 2021 cars pass most often (87.3%) and 2012 cars least (70.2%), though most of that gap is age. Every figure here is what testers recorded, not what owners reported.

A “common problems” list is usually a forum thread: whoever had the worst day writes the longest post. This one is built the other way, from every Seat Mii that took an MOT last year and what the tester wrote down. It cannot see breakdowns, electrical gremlins or a gearbox that hunts, because the test does not check those. What it can see, it sees across 18,638 cars.

By component

What wears out on a Mii

Failure items grouped by the part behind them, with the share of failed Seat Mii tests recording each. A part can carry several DVSA wordings; they are listed beneath. One failed test often lists several items, so shares do not sum to 100%.

  1. 1.Tyres

    27.1% of failed tests

    Tread wears with every mile and sidewalls age and take kerb damage; the legal minimum is 1.6 mm across the central three-quarters.

    • “Tyre tread depth not in accordance with the requirements” — explained10.6%
    • “A tyre seriously damaged” — explained5.2%
    • “A tyre seriously damaged” — explained5.1%
    • “A tyre cords visible or damaged” — explained3.4%

    Across all cars, tyres failures run at a similar rate at every age, because they are replaced on a cycle rather than wearing out with the car. How long tyres last

  2. 2.Catalytic converter and emissions

    24.2% of failed tests

    Catalysts degrade with age and misfires; a failed emissions reading is usually the catalyst, a lambda sensor or a tired engine.

    • “Lambda coefficient outside the default limits or the range specified by the manufacturer” — explained7.3%
    • “Emissions levels exceed default limits” — explained4.2%
    • “Emissions levels exceed the manufacturer's specified limits” — explained3.7%
    • “Emissions levels exceed default limits” — explained2.5%

  3. 3.Bulbs and lamps

    18.4% of failed tests

    Filament bulbs simply expire; the cheapest and most avoidable failure on the test, and one of the most recorded.

    • “A lamp missing, inoperative or in the case of a multiple light source more than 1/2 not functioning” — explained7.5%
    • “A rear registration plate lamp or light source missing or inoperative in the case of a single lamp or all lamps” — explained5.7%
    • “A headlamp or light source missing, inoperative or more than ½ not functioning in the case of LED” — explained3.6%
    • “Stop lamp missing, inoperative or in the case of a multiple light source more than 1/2 not functioning” — explained1.7%

    Across all cars, bulbs and lamps failures climb 15-fold between three and fifteen years, peaking at 15+ years. How long bulbs and lamps last

  4. 4.Brake callipers

    16.8% of failed tests

    Pistons and slider pins seize with corrosion and disuse, dragging a brake on or leaving one side weak; shows as imbalance on the roller tester.

    • “Significant brake effort recorded with no brake applied indicating a binding brake” — explained10.4%
    • “Brakes imbalance across an axle such that the braking effort from any wheel is less than 70% of the maximum effort recorded from the other wheel on the same axle.” — explained4.1%
    • “Significant brake effort recorded with no brake applied indicating a binding brake” — explained2.3%

  5. 5.Suspension bushes

    14.4% of failed tests

    Rubber bushes in arms and links perish and split with age and load; the most recorded suspension item on the test.

    • “A suspension pin, bush or joint excessively worn” — explained11.2%
    • “A suspension joint dust cover missing or no longer prevents the ingress of dirt etc” — explained3.1%

    Across all cars, suspension bushes failures climb 20-fold between three and fifteen years, peaking at 15+ years. How long suspension bushes last

  6. 6.Wiper blades and washers

    12.4% of failed tests

    Rubber blades harden and split within a year or two; washer jets block and pumps fail.

    • “Wiper blade missing or obviously not clearing the windscreen” — explained7.3%
    • “Windscreen washers not working or not providing sufficient fluid to clear the windscreen” — explained2.7%
    • “Windscreen washers not working or not providing sufficient fluid to clear the windscreen” — explained2.4%

    Across all cars, wiper blades and washers failures run at a similar rate at every age, because they are replaced on a cycle rather than wearing out with the car. How long wiper blades and washers last

  7. 7.Parking brake

    9.9% of failed tests

    Cables stretch and seize, rear shoes and adjusters wear; efficiency is measured on the roller tester against the vehicle's weight.

    • “Parking brake efficiency below minimum requirement” — explained4.6%
    • “Parking brake lever has excessive movement indicating incorrect adjustment” — explained3.8%
    • “Parking brake inoperative on one side” — explained1.6%

  8. 8.Coil springs

    7.5% of failed tests

    Springs fatigue and corrode where the coating chips, then snap, often at the bottom coil; potholes and salt accelerate it.

    • “A spring or spring component fractured or seriously weakened” — explained7.5%

    Across all cars, coil springs failures climb 20-fold between three and fifteen years, peaking at 12–14 years. How long coil springs last

By registration year

Mii problems by year

MOT pass rate for Seat Mii cars by the year they were first registered, from years with at least 1,000 tests. Every car in a row is the same age, so the rate is raw. Neighbouring years differ by a year of wear — a step of a point or two is age. The step at 2017, up 6.1 points, is larger than wear explains and most likely marks a change in the cars themselves.

201270%
201370%+0.2
201475%+4.9
201574%−1.5
201674%+0.3
201780%+6.1
201883%+2.5
201984%+1.4
202187%+3.2

Last column: change on the previous year, in points. Years registered within the last three years have not yet been tested and are not shown.

Before it fails

What testers warn Mii owners about

Advisories are the same parts a year or two earlier. Share of all Seat Mii tests carrying an advisory for each.

Go deeper

The rest of the Mii file

Common questions

What are the most common problems with a Seat Mii?
Going by what fails the MOT, tyres (recorded in 27.1% of failed tests), catalytic converter and emissions (recorded in 24.2% of failed tests), and bulbs and lamps (recorded in 18.4% of failed tests). These are the items testers actually record across 18,638 tests, not reports from owners, so they describe what goes wrong at the point where it has to be fixed.
Which Seat Mii year is the most reliable?
By MOT pass rate, Seat Mii cars registered in 2021 do best at 87.3% and 2012 cars do worst at 70.2%. Most of that ordering is simply age — newer cars pass more — so read the year table for steps larger than a couple of points between neighbouring years; those mark a change in the cars rather than the calendar. The largest such step is at 2017, up 6.1 points on 2016.
Is the Seat Mii reliable?
Adjusted for the age and mileage of the cars tested, the Seat Mii passes 74.3% of MOTs against a national 72.3%. An MOT measures roadworthiness on the day rather than breakdowns or running costs, so this is a reliability measure in the narrow sense: how often the car meets the legal standard, and what stops it.
What should I check when buying a used Seat Mii?
Start with what fails most: tyres, bulbs and lamps, suspension bushes. Then read the car's own MOT history for advisories carried year to year without repair — the model tells you what to expect, the history tells you whether this owner dealt with it.

Analysis built from DVSA data of