The structure of the tyre has been damaged. A bulge means the internal cords have broken and only the rubber is holding the air in at that point; exposed cord means the outer rubber has worn or been cut through to the carcass.
Wording as it commonly appears on an MOT certificate: “Tyre has a cut, lump, bulge or tear caused by separation or partial failure of its structure”.
What does it mean?
The structure of the tyre has been damaged. A bulge means the internal cords have broken and only the rubber is holding the air in at that point; exposed cord means the outer rubber has worn or been cut through to the carcass.
How serious is it?
Usually recorded as: Dangerous. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- A hard pothole or kerb strike, which pinches the sidewall against the rim and snaps the cords
- Long-term under-inflation flexing the sidewall
- Kerbing that scrubs the sidewall rubber away
- Impact damage that only shows up as a bulge days or weeks later
What a mechanic would check
Run a hand slowly over both sidewalls with the wheel turning — bulges are easier to feel than see. I'd also check the inner sidewall, which you can't see with the wheel on the car, and look at the wheel rim for a matching flat spot from the same impact.
What might need replacing
- That tyre replaced — structural damage is never repairable
- The wheel checked for buckling from the same impact
Can you still drive it?
This is normally recorded as Dangerous, and it deserves it. A structurally damaged tyre can let go at speed with no warning. Don't drive on it — change to the spare or get it recovered.
Repair cost
The cost is a tyre, plus a wheel if the same impact buckled the rim. Never let anybody talk you into a repair on sidewall or structural damage; it isn't repairable and no reputable fitter will attempt it.
Before the retest
Check every tyre, not just the one that failed — the legal minimum is 1.6mm across the central three-quarters of the tread, all the way round. Make sure tyres on the same axle match in size and construction, and that all wheel nuts or bolts are present and tight. Set pressures to the figure on the door shoulder or in the handbook the morning of the retest.
Retest rules
If the vehicle stays at the test centre for repair and is retested within 10 working days, a partial retest is often free; some items qualify for a free retest if you bring the car back by the end of the next working day. The exact conditions are set out on GOV.UK, so confirm with the test centre before you leave. A major or dangerous failure means the car cannot legally be driven away unless the previous MOT is still valid and nothing dangerous was recorded — check your certificate first.
Related help
More tyres and wheels failures · MOT failure decoder · Ask a mechanic about your certificate
This is general guidance from a UK mechanic, not a substitute for a physical inspection. Only the tester who inspected your vehicle can confirm what was recorded and why.