Brake Discs and Your MOT
Brakes account for roughly 10% of all MOT failures. Here is exactly what the tester checks on your discs, what advisory vs failure means, and your rights if the garage says you need new discs.
What the MOT Tester Checks on Brake Discs
The MOT tester inspects brake discs both visually and mechanically. The visual inspection happens with the wheels on (looking through the spokes) and sometimes with wheels removed if there is concern. The mechanical test uses a roller brake tester.
| Check | What It Means | Pass/Fail Threshold |
|---|---|---|
| Disc thickness | Measured with a vernier caliper against the manufacturer's minimum (stamped on the disc) | Below minimum = major defect |
| Surface scoring | Depth of grooves cut into the disc face by worn pads or debris | Deep scoring = advisory or major depending on depth |
| Cracking | Cracks in the disc surface from heat stress | Any significant cracking = major defect |
| Edge lip | Raised edge around the disc where the pad does not reach | Excessive lip = advisory or major |
| Corrosion | Rust on the disc friction surface (common on parked cars) | Light surface rust is normal. Heavy pitting = advisory |
| Heat damage | Blue/purple discolouration from overheating | Evidence of overheating = advisory or major |
| Roller brake test | Measures braking force and balance between left and right wheels | Imbalance above threshold = major defect |
The Roller Brake Test
During the MOT, the car is driven onto rollers that spin each wheel. The driver presses the brake pedal and the machine measures how much braking force each wheel produces. The tester compares left vs right on each axle.
Worn discs cause imbalance because one disc may be thinner or more scored than the other, producing less friction. If the imbalance between left and right exceeds the threshold (typically 30% difference), it is a major defect.
This is why brake discs must always be replaced in pairs per axle. A new disc on one side with a worn disc on the other will fail the roller brake test.
Advisory vs Major Defect
Advisory
- Discs are wearing but still above minimum thickness
- Light scoring that does not affect braking performance
- Minor lip developing around the edge
- No retest needed - the car passes
- Plan replacement within the next few months
- Appears on the MOT certificate as an advisory note
Major Defect (Failure)
- Discs below manufacturer's minimum thickness
- Deep scoring or cracking that affects braking
- Excessive imbalance on the roller brake test
- Must be repaired before the car can be driven legally
- Retest required (free if done within 10 working days at the same station)
- Dangerous defect classification if severely worn
Common MOT Disc Failure Reasons
| Failure Reason | What It Means | Typical Fix Cost |
|---|---|---|
| Disc below minimum thickness | Worn beyond safe use. Must be replaced. | £150 - £350 per axle |
| Excessively scored disc | Deep grooves from worn pads grinding metal-on-metal | £150 - £350 per axle |
| Cracked disc | Heat stress cracking. Safety risk. | £150 - £350 per axle |
| Brake imbalance (roller test) | One side weaker. Usually uneven disc wear or seized caliper. | £150 - £500+ per axle |
| Disc significantly corroded | Deep pitting on friction surface from prolonged parking | £150 - £350 per axle |
Your Rights After an MOT Brake Failure
- 1.You do not have to use the testing station's garage for the repair. You can take the car elsewhere.
- 2.The garage cannot hold your car. It is your property.
- 3.You have 10 working days to get a free partial retest at the same station (brakes only). After that, you pay for a full retest.
- 4.You can challenge the MOT result with the DVSA if you believe the failure is incorrect. Call their helpline or use the online appeal form.
- 5.If the car is driven home with a major defect, you are technically driving an unroadworthy vehicle. Your insurance may not cover you in an accident.
- 6.Get a second opinion if you are unsure. Another garage can inspect the discs and give you their view before you commit to the repair.
Should You Replace Discs Before the MOT?
If your discs are borderline, replacing them before the MOT is often the smarter move. A pre-MOT brake check (free at Kwik Fit, Halfords, and most chains) will tell you whether they are likely to pass.
The advantage of replacing beforehand: you can shop around for the best price, choose your preferred garage, and avoid the rush of needing repairs done within the 10-day retest window.
The disadvantage: you might replace discs that would have scraped through. But given that borderline discs will need replacing soon anyway, the peace of mind is usually worth it.
MOT Brake Failure Statistics
~10%
of all MOT failures are brake-related
1 in 20
vehicles get a disc-specific advisory or failure
30%
imbalance threshold that triggers a failure
Based on DVSA annual MOT data. Brakes are consistently one of the top three failure categories alongside lighting and tyres.
Related Guides
Warning Signs
8 symptoms your discs need replacing
Disc Lifespan
How many miles do discs last?
UK Garages
Where to get the best price on repairs
Cost Calculator
Estimate what your repair will cost
For a full guide to MOT costs, see MOTCost.com.