The disc has lost enough thickness, or picked up enough scoring and pitting, that the tester judged it no longer fit for purpose. A pronounced lip around the outer edge is usually what triggers this — that lip is the original thickness, and everything inside it is what's worn away.
Wording as it commonly appears on an MOT certificate: “Brake disc significantly and obviously worn”.
What does it mean?
The disc has lost enough thickness, or picked up enough scoring and pitting, that the tester judged it no longer fit for purpose. A pronounced lip around the outer edge is usually what triggers this — that lip is the original thickness, and everything inside it is what's worn away.
How serious is it?
Usually recorded as: Major. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- High mileage on the original discs, especially if pads have been changed once or twice without changing discs
- Corrosion pitting on a car that's stood for long periods or does short damp journeys
- Running pads down to metal, which cuts grooves straight into the disc face
- A sticking caliper keeping one pad in constant contact
What a mechanic would check
Micrometer on the disc face in a few places, compared against the minimum thickness that's usually cast or stamped into the disc hub. I'd also run a finger over the friction surface for grooves and check both sides — the inboard face is nearly always worse and is the one people never look at.
What might need replacing
- Discs and pads as a pair on that axle
- Caliper attention if the wear pattern is uneven side to side
Can you still drive it?
Recorded as a failure, so it needs sorting before the car goes back on the road. Badly worn discs reduce how much heat the brakes can shed, which shows up as fade on a long descent exactly when you don't want it.
Repair cost
Discs and pads together cost more than pads alone, but fitting new pads to worn discs is false economy — the new pads never bed properly and you're back in six months. Get the actual measurement before deciding.
Before the retest
Brake work is usually done in axle pairs — pads and discs are replaced on both sides of the same axle so the car pulls up straight on the roller tester. After new pads or discs, drive gently for the first hundred miles or so if you can, because brakes need bedding in before they reach full efficiency. Keep the old parts or the invoice; a tester will not ask for them, but they settle any argument about what was actually replaced.
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 brakes 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.