How to Tell If a Windscreen Chip Fails Your MOT (and Repair Cost)

• A windscreen chip fails the MOT if it is wider than 10mm in the driver’s zone, a strip 290mm wide centred on the steering wheel, or wider than 40mm anywhere else the wipers sweep.
• A repair is usually possible while a chip is small and clean. Prices run from about £25 ($33) at an independent specialist to £225.98 ($294) at a national chain with no glass cover, while a full replacement costs roughly £260 ($338) to £1,350 ($1,755).
• Fix a chip within days. Dirt, moisture, vibration and a blast of heater on a cold morning can turn it into a crack that forces a replacement.

How to Tell If a Windscreen Chip Will Fail Your MOT, and When a Repair Still Works

A chip in the glass in front of the driver fails the MOT once it is bigger than 10mm across. A chip anywhere else in the area your wipers sweep fails at more than 40mm. Those two numbers come straight from the official MOT inspection manual, and they decide whether a small star or bullseye mark is a nuisance you can leave for a few weeks or a failure that needs fixing before the test.

Measure the chip with a ruler before you do anything else. If it sits inside the driver’s zone and is bigger than 10mm, book a repair or replacement now. If it sits elsewhere and is under 40mm, you will pass the damage check, but get it looked at soon. Small chips rarely stay small.

The 10mm and 40mm rules that decide an MOT pass

The MOT inspection manual published on GOV.UK splits the windscreen into two areas. The first is the part directly in front of the driver. The manual describes it as the swept area of the wipers, 290mm wide and centred on the steering wheel. Damage there is a fail if it is more than 10mm in diameter. The second area is everything else the wipers sweep. Damage there is a fail if it is more than 40mm in diameter.

The manual adds a condition that surprises many drivers. A tester should fail the screen for damage only if it significantly affects the driver’s view of the road. The manual also tells testers not to take into account whether the driver is tall or short. In practice the size limits act as the working rule, and a chip bigger than the limit in the wrong place is treated as a problem with the view.

Defects are graded. Damage that spoils the view of the road is a major defect, which fails the test. Damage so bad that visibility through the swept area is seriously affected is classed as dangerous, which means the car should not be driven at all until it is fixed. A minor defect is damage that does not adversely affect the driver’s view. That one does not fail the test, but it tells you the glass is worth fixing soon.

Repairs get separate treatment. The manual says a repaired screen is judged on whether the repair interferes with vision. In its words, an “invisible” or barely detectable repair, finished flush with the surrounding glass, does not count as damage. That is why a well-done resin repair on a chip can turn a likely fail into a clean pass.

Autoglass gives the same two figures on its MOT page. It also points out that the test covers more than the glass. A car can fail if the washers do not supply enough liquid, or if the wipers are missing, loose or worn. A chip is only one of three things a tester looks at on the front screen.

Why a small chip turns into a crack

Glass is stiff, and a chip leaves a tiny weak point surrounded by stress. Every pothole, door slam and change in temperature pulls on that weak point. Nick Reid, head of automotive technology at Green Flag and a fellow of the Institute of the Motor Industry, says dirt and moisture can work into a chip, and that vibration and temperature changes can then spread it into a crack. A crack often means replacement rather than repair.

Haynes makes the same point about heat. Using the heater at full blast on a frosty windscreen creates thermal shock, a fast difference in temperature across the glass. A chip that has sat quietly for weeks can run across the screen in minutes when the cold glass heats unevenly. October is when this starts to happen. The first frosts arrive and drivers switch on the demister without thinking.

The practical rule is to treat a chip as time-limited. Clean chips repair well. The resin bonds to bare glass. Old chips filled with dirt and water often cannot be fixed, according to Haynes, so a chip you ignore for a month is less likely to qualify for the cheap option than the same chip fixed this week.

There is also a reason to act that has nothing to do with the MOT. Haynes notes that the Highway Code expects you to have a full view of the road, and damage below the MOT limits can still cause problems after an accident. A chip that is legal on test day is not a guarantee of a clean claim if an insurer later argues it blocked your view.

Autoglass says it repairs chips to British Standard BS AU242b:2022, leaving the repair flush with the glass and almost invisible. Some chips cannot be repaired, and the company says cracked or badly chipped screens need replacing before the MOT. Where the chip is, how big it is and what type it is all affect the decision.

How much a windscreen repair or replacement costs

Prices swing more widely than most motoring jobs, so it pays to get at least two quotes before you book. At the cheap end, Green Flag’s Reid puts a repair from an independent specialist at around £25 ($33) and about half an hour of work, though you have to take the car to them. A DIY resin kit costs about £10 ($13), and he notes that some people get on well with them while others find them frustrating.

At the other end, Autoglass lists a single chip repair at £225.98 including VAT ($294) for drivers with no glass cover. Additional chips on the same visit are quoted on request. The gap between that figure and an independent specialist’s price is wide, so a quote from each is worth the phone calls.

If you have glass cover on your car insurance, the maths changes. Reid says insurers typically charge an excess of about £50 ($65) for a repair, with some policies charging nothing, and the job takes about half an hour, often at a place of your choosing. Autoglass says that with glass cover you usually pay only your excess and your no claims bonus is not reduced. Confused.com says the position on no claims bonus depends on the insurer, and that some insurers repair chips free while others charge fees. Check your policy wording before you ring anyone.

Replacement is a different scale of bill. Confused.com quotes Checkatrade figures of around £260 ($338) for a small budget car and around £1,350 ($1,755) for a high-end car, and the cost depends heavily on the make and model. The gap between a £25 repair and a £1,350 replacement is the best argument for fixing a chip early.

Policies differ on how many repairs they allow. Confused.com says some policies allow unlimited windscreen repairs while others cap claims each year, and that you must declare windscreen claims at renewal. Failing to declare them could invalidate your policy, so keep the paperwork from any repair.

Edge cases: cracks, edges, old repairs and the cost of a failed MOT

Not every mark is a chip. Haynes says star cracks from an impact are likely repairable, while larger cracks, especially stress cracks from temperature changes or body flexing, are likely beyond repair. A crack that reaches the edge of the windscreen affects the structure of the glass, and it needs replacement. Scratches can also require a new screen, even when they look minor, as they can impair vision in low sun.

Position inside the swept area changes the repair limit and the MOT limit. Haynes passes on Autoglass guidance that splits the screen into four zones. Zone B, swept by the driver’s wipers, allows chips up to 1.5cm. Zone C, the passenger side, allows up to 2.5cm. Zone D, out to the edges, allows up to 4cm, but damage within 4cm of the edge needs a replacement. Green Flag’s Reid gives the general rule that chips under 40mm are repairable if they are not in the MOT-critical area, and that chips in the driver’s zone are repairable only at 10mm or less.

If your car fails on the screen, the cost of getting back to a pass is the repair or replacement itself. Driving without a valid MOT can bring a fine of up to £1,000 ($1,300) and can invalidate your insurance, according to Autoglass, and you need a valid MOT to tax a car over three years old. That makes a screen failure more expensive than the glass alone.

If your chip dates from a motorway trip last summer, the MOT is the point where a tester will measure it. You can renew up to one month before your current MOT expires, which gives you a window to fix the screen without losing your renewal date.

One last warning concerns temporary fixes. Clear tape or a sticker over a chip keeps dirt out for a day or two, but it is not a repair, and a tester will measure the damage underneath it. If the chip is in your direct line of sight and bigger than 10mm, do not wait for the weekend.

Sources

• GOV.UK: 3. Visibility
• Autoglass: UK MOT Rules for Windscreens
• Autoglass: Affordable windscreen repair cost
• Green Flag: Expert advice: What to do with a chipped windscreen
• Haynes: Will a windscreen crack cause your car to fail its MOT?
• Confused.com: Windscreen cover