Our own MOT research
Which MOT advisories actually turn into failures?
Our own analysis matched 124,130 cars across two years of MOT tests in the Cambridge postcode area. Every advisory category predicted a failure in the same category the following year, and they did not do it equally. Brakes is the one worth dealing with soonest: not the biggest jump in the table, but the system with the least margin in it when you get it wrong.
Brakes: the one with the least margin
Of the cars advised on their brakes, 9.25% failed on brakes the following year, against 3.91% of those not advised. That is more than double, on the second largest group in the table: 26,358 cars were advised on brakes.
Brakes are also where being late costs the most. A worn suspension or steering component usually announces itself through the wheel long before it fails a test. Brake pads that are down to the backing plate do not give you much notice, and what they do to the discs underneath turns a cheap job into a more expensive one.
Steering shows the biggest jump, and it is not work we do
The largest effect in the table is steering: 17.02% of the 1,798 cars advised on steering failed on steering the next year, against 1.59% of the 122,332 that were not. Some of that gap is age rather than the advisory, because cars carrying one tend to be older, but it is still the biggest of the five.
Steering is not on our price list, and we would rather say so than write around it. If that is what your certificate says, take it seriously, because on these numbers it is the advisory least worth leaving. What we can do is read the certificate with you and tell you what the rest of it means, which is usually where the brake and service items are hiding.
| Advised on | Failed next year after an advisory | Failed next year without one | Cars advised |
|---|---|---|---|
| Steering | 17.02% | 1.59% | 1,798 |
| Lamps and electrics | 27.91% | 11.27% | 3,458 |
| Suspension | 17.21% | 7.06% | 19,527 |
| Brakes | 9.25% | 3.91% | 26,358 |
| Tyres | 11.7% | 6.44% | 35,595 |
What the table is really telling you
Every category points the same way, which is the point. An advisory is not a note about the past. It is the earliest sighting of the thing that will eventually stop the car passing, written down by somebody who had it on a ramp.
The categories differ in how much warning they give you and in what it costs to be late. Lamps and electrics are cheap to put right and cheap to ignore. Steering and brakes are neither.
What this does not show
This is one year in one postcode area, not a census and not a national picture. It is directional, and a category with fewer cars behind it carries less weight.
The table is raw, the seven times figure is age-adjusted. They are measuring the same thing with and without an allowance for the age of the cars, and the adjusted one is the more honest comparison.
An advisory does not cause a failure. Both follow from the condition of the car. The advisory is the early sighting of the same wear, not the reason for it.
Not every car comes back. About a third of the cars tested in the earlier year did not appear in the later one. They were sold, scrapped, untested or re-registered, and this data cannot tell those apart. The matched set is 124,130 cars.
CB is not Cambridgeshire. The postcode area takes in Newmarket and Haverhill in Suffolk and Saffron Walden in Essex, and leaves out Peterborough, Huntingdon, March and Wisbech.
If your certificate carries a brake advisory, the cheap moment to deal with it is now rather than after the next test, and the place to start is a brake check or replacement at your home or work.
Got a brake advisory?
Send us what the certificate says and we will tell you whether it needs doing now or can wait, and what it costs either way. Brakes are priced per axle and agreed before anything starts, at your home or work.
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Related: our full MOT research, your car passed but was still advised, MOT advisories explained, mobile brake repair.