The MOT Files

MOT defect file

Tyre damaged or deteriorated

Last updated

Quick answer

The tyre has a cut, bulge or perishing sidewall. This cannot be repaired — the tyre needs replacing.

Typical cost
£70–£160 per tyre
DIY viability
Garage job
Tyre wall cut through to the reinforcing cordA straightened section through a tyre wall shows tread blocks joined continuously to the outer rubber, a distinct reinforcing ply band containing closely spaced parallel cords, and the inner liner below. A deep V-shaped cut passes through a tread block and all of the covering rubber until it reaches and exposes the cord band. The remaining tread blocks do not alter the result: visible or damaged ply or cord is a major MOT defect.Remaining tread blocksCord exposed by cutReinforcing ply and cordInner linerExposed cord = major defect, whatever the tread depth
If any tyre ply or cord is exposed or damaged, the tyre has a major defect regardless of its remaining tread depth. The curved tyre wall is straightened and the layers and cut are enlarged here for clarity, not shown to scale.

What you might notice

  • A bulge or egg-shaped swelling in the sidewall, usually appearing after a pothole or a kerb strike
  • Fine cracking across the sidewall or between the tread blocks on a tyre that has aged rather than worn
  • A slow puncture that keeps returning, or a tyre needing topping up every couple of weeks
  • Nothing visible from the kerb — inner sidewall damage only shows itself on full lock or from underneath

What causes it

  • Kerbing or a pothole impact snapping some of the casing cords, so air pressure pushes the sidewall outwards
  • Age: rubber hardens and crazes after roughly six to ten years regardless of mileage, which is why low-mileage cars fail on tyres
  • Persistent under-inflation flexing the sidewall until the structure breaks down internally
  • Contact with oil or fuel, or rubbing against a displaced arch liner or a worn suspension component

Advisory or failure?

Superficial crazing of the rubber that does not reach the casing is generally recorded as an advisory. Damage becomes a major defect once the tyre is cut, torn or perished more seriously than that. It is a dangerous defect — and one of the most common reasons a car is failed as dangerous — where ply or cord is exposed or damaged, where there is a lump, bulge or tear caused by the casing separating, or where a cut is longer than 25mm or 10% of the tyre's section width, whichever is greater, and deep enough to reach the cords.

Can you fix it yourself?

Garage job

Fitting requires a tyre machine and the wheel must be rebalanced afterwards, so the replacement itself is never a home job. Inspecting is, and it is worth doing: turn the wheels to full lock and look at the inner sidewalls, where kerb damage and rubbing hide, and check the four-digit date code moulded into the sidewall — 3419 means the thirty-fourth week of 2019. Know what cannot be fixed: a sidewall cut or bulge is never repairable, and only a puncture within the central three-quarters of the tread can be plugged, to BS AU 159.

What it costs

A budget tyre in a common 15 or 16 inch size is around £55 to £75 fitted, balanced and with a new valve; mid-range and premium in the same size run £85 to £130. You reach the top of the band with larger diameters, low-profile and run-flat fitments, and where a tyre pressure sensor has to be serviced or replaced at the same time. Replace in pairs across an axle if the surviving tyre is also part-worn, and where the damage came from rubbing rather than an impact, have the alignment and suspension checked or the new tyre will go the same way.

What happens if you ignore it

A damaged carcass tends to fail suddenly rather than deflate gradually, and a front tyre letting go at speed takes the steering with it. The legal position matches that for a bald tyre: up to £2,500 and three penalty points, applied per tyre, so a full set can put a licence past the twelve-point threshold in a single stop. An insurer presented with a claim involving a tyre the owner knew was damaged has firm ground to argue from.

Common questions

Is a bulge in a tyre sidewall an MOT failure?
Yes, and it is graded dangerous rather than simply major. A lump, bulge or tear caused by the casing separating means the structure of the tyre has broken down, as does exposed or damaged ply or cord. This is one of the most common reasons a car is failed as dangerous, and the tyre has to be replaced before the car is driven.
Can a damaged tyre be repaired?
Only some punctures. A repair is permitted to BS AU 159 where the puncture falls within the central three-quarters of the tread. A cut, tear or bulge in the sidewall is never repairable, because the casing cords that carry the load have been damaged and no plug or patch restores them.
How old is too old for a tyre?
Rubber hardens and crazes after roughly six to ten years regardless of mileage, which is why low-mileage cars so often fail on tyres. The four-digit code moulded into the sidewall gives the week and year of manufacture, so 3419 means the thirty-fourth week of 2019. Superficial crazing that does not reach the casing is generally only an advisory.
How do I check for tyre damage I cannot see?
Turn the steering to full lock and look at the inner sidewalls, which is where kerb damage and rubbing hide and where nothing shows from the kerb. Run a hand around the tread and sidewall feeling for lumps, and check the date code while you are down there. A puncture that keeps returning is also worth investigating properly.
How much does it cost to replace a damaged tyre?
A budget tyre in a common 15 or 16 inch size is around £55 to £75 fitted, balanced and with a new valve, while mid-range and premium in the same size run £85 to £130. Larger diameters, low-profile and run-flat fitments cost more, as does servicing or replacing a tyre pressure sensor at the same time.

Cars this happens to most

Of every model with at least 2,000 tests in the DVSA data, these record tyre damaged or deteriorated on the largest share of their MOTs. Across all of them the typical rate is 2.7% of tests.

  1. BMW X78.4%

    217 times in 2,595 tests

  2. Mercedes-Benz Eqb8.2%

    262 times in 3,194 tests

  3. BMW I48.0%

    554 times in 6,958 tests

  4. Tesla Model Y7.7%

    2,689 times in 34,928 tests

  5. Polestar 27.1%

    993 times in 13,952 tests

  6. Mercedes-Benz Eqc6.7%

    834 times in 12,524 tests

  7. Audi Q4 E-Tron6.5%

    695 times in 10,679 tests

  8. Mini John Cooper Works5.8%

    1,282 times in 22,101 tests

A high rate here is a property of the fleet as much as the car: these models skew old, and almost everything fails more with age. It is a reason to check the item on a used example, not proof the model is badly built. Models with fewer than 2,000 tests are left out because a rate drawn from a few hundred cars moves too much to rank.

Pass rates for every make and model

This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.

Has this been recorded on your car?

Check the full MOT history for any UK registration.

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