Will Cracked Tyres Fail an MOT? The Line the Tester Works To

Surface cracking on its own is normally recorded as an advisory rather than a fail. What fails a tyre is damage that reaches the ply or cords, or a bulge showing the structure has already given way. The tester has a defined method for deciding which of those a crack is, and it involves opening the crack up.

Key takeaways:

• Light surface crazing passes, usually with an advisory.
• Cracks or cuts that reach the cords fail.
• A bulge is a dangerous defect, and the car should not be driven until the tyre is replaced.

What the Tester Is Looking For

The tyre section of the MOT inspection manual is short and specific. It sets out what counts as a defect and how a tester decides, and the word crack does not decide anything on its own.

Cracking Is Not on the List

The manual’s tyre defects cover size and type, condition and fitment, tread depth, and damage. Cracking does not appear anywhere as a defect in its own right.

That is why an ageing tyre covered in fine crazing can pass. The tester is not marking a tyre down for looking old. The question is what the cracking has reached: whether it stays in the surface rubber, or whether it has opened a way through to the ply and cords that give the tyre its strength.

Where the cracking is visible but stays in the surface, the usual outcome is an advisory. That puts it on the certificate and on the car’s MOT history, without failing the car.

The Cut Test, Step by Step

The MOT inspection manual sets out three ways a cut fails, and a tester applies the same method to a crack.

First, any ply or cord that can be seen without touching the tyre is a fail. Second, ply or cord that becomes visible once the rubber is folded back, or the cut is opened with a blunt instrument, is a fail whatever the size of the cut. Third, where the cut runs to more than 25mm or 10 per cent of the section width, whichever is greater, and the cords can be felt but not seen when it is opened, that is a fail too.

The 25mm figure belongs to that third case alone. It is not a general size limit, and a crack of any length fails if it exposes the cords. The manual also tells testers to be certain it is cords they can feel rather than a stone or other foreign object, and to pass and advise where they are not sure.

Major, Dangerous and What Each Means

DVSA records a cut beyond those requirements, deep enough to reach the ply or cords, as a major defect. A tyre with a lump, bulge or tear caused by separation or partial failure of its structure, including any lifting of the tread rubber, or with cords exposed or damaged, is a dangerous defect.

The difference shows when it is time to leave the test station. A major defect fails the car, but if the old certificate still has time left on it, the car can be driven away to be repaired. A dangerous defect fails it with an instruction attached: the car should not be driven until the tyre has been replaced, whatever the certificate says.

Tyre faults fill several places among the faults that fail cars. In 2024, tread below the limit appeared on 759,464 failed tests, cords visible or damaged on 443,729, and a tyre seriously damaged on 436,782. Tread below 1.6mm in a continuous band across the central three-quarters of the tyre, around its whole circumference, is itself a dangerous defect.

Why Tyres Crack, and When It Becomes Serious

Rubber does not last forever. The reason a tyre has cracked, and where on the tyre the cracks sit, decide how seriously to take them.

Age, Sunlight and Standing Still

Tyre rubber hardens with time. Ultraviolet light, ozone in the air and repeated heating and cooling all draw flexibility out of it, and the surface starts to craze into a fine network of cracks.

Low-mileage cars and caravans suffer most. Their tyres age on the calendar rather than the odometer, so a tyre with plenty of tread left can be cracking on a car that rarely leaves the drive. Standing in one position for long spells, out in the sun, adds to it.

Under-inflation speeds the process up. A soft tyre flexes harder at the sidewall with every turn of the wheel, and that flexing opens cracks that would otherwise stay shallow. Checking pressures regularly is the cheapest protection a tyre gets.

Where the Crack Sits

Cracking at the bottom of the tread grooves is common on older tyres and is often shallow. Cracking on the sidewall is the type most drivers notice, and on an ageing tyre it is usually surface crazing.

The shoulder, where the tread meets the sidewall, is the area to take most seriously. It flexes hardest, carries the load in corners, and is where separation between the tread and the structure beneath tends to show first.

A tyre with cracks opening at the shoulder is worth replacing before the test rather than arguing about on the day. The tester’s question is whether the damage reaches the cords, and at the shoulder the answer can change quickly.

What to Do About It

A driver can see most of what a tester sees, without any tools. The part of the test that involves opening a crack belongs to the tester, not the owner.

Checking Your Own Tyres

Turn the steering to full lock so the inner shoulders and the back of the tread come into view, and look in good daylight. Check the sidewalls, the shoulders and the bottom of the tread grooves, then move the car a short distance to see the part of each tyre that was resting on the ground.

Fine, shallow crazing that looks the same all the way round is the ageing pattern. Cracks that look deeper at the shoulder or sidewall, that open wider where the tyre flattens under the car’s weight, or any lump or bulge at all, need a professional look.

Do not poke at a crack with a screwdriver or anything else. Opening a crack is a tester’s method, not a driver’s, and it tells an owner very little. A bulge means the tyre is replaced before the car goes anywhere.

Advisory Now, Fail Next Year

An advisory for cracking is a warning with a 12-month clock on it. The tyre will be a year older at the next test, and the cracks will have had a year to deepen.

On younger cars, tyres are the defect most likely to be found. Tyres lead every age band up to six years old, appearing in 5.03 per cent of tests on cars under three, 4.81 per cent at three and 5.45 per cent at four to six. Across all 33,061,250 car tests in 2024, tyre defects appeared in 6.43 per cent, and tread approaching the limit was recorded as an advisory on 5,308,869 tests, 16.06 per cent of the total.

Dealing with an advisory before the next test means choosing the tyre and the garage in your own time. Leaving it until the day means accepting whatever the test station has in stock, or a fail and a retest.

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