MOT Advisories Explained: What They Mean and When to Worry

Every year, millions of UK drivers get their MOT results back and find themselves staring at a list of 'advisories' with no idea what they actually mean. Is that tyre wear something you need to fix tomorrow, or can it wait six months? What's the difference between an advisory and a minor defect? And when does an issue cross the line into dangerous territory?

We've analysed 1,498 MOT tests across 1,112 vehicles to show you exactly what these categories mean in practice, which defects testers flag most often, and when you should actually worry. The confusion is real: the MOT system uses four different severity levels, and understanding them could save you hundreds of pounds in unnecessary repairs or help you avoid buying a dangerous car.

What Are the Four MOT Defect Categories?

Since May 2018, the MOT system has used four distinct categories to grade vehicle defects. Here's what they actually mean:

Advisory: An observation about something that might need attention in future but doesn't affect your vehicle's roadworthiness right now. Your car passes the MOT with advisories. Common examples include tyres approaching the legal limit or slight corrosion starting to appear.

Minor defect: A fault that has no significant effect on vehicle safety or the environment. Your car still passes with minor defects, but you should address them. These are rare in practice because most issues that need fixing fall into the next category.

Major defect: A fault that could affect vehicle safety, puts other road users at risk, or has a significant impact on the environment. Your car fails the MOT. You can have it repaired and retested. Examples include significantly worn brake pads or broken suspension components.

Dangerous defect: An immediate risk to road safety or serious environmental impact. Your car fails the MOT and is legally unroadworthy. You cannot drive it until the defect is fixed. Think completely worn brake pads, severely damaged tyres, or steering that's about to fail.

Which Defects Show Up as Advisories Most Often?

The data tells you exactly what MOT testers notice most frequently. Across the commercial vehicles in our sample, one advisory dominates: tyres worn close to the legal limit or worn on the edge. This appeared in 17.3% of tests for Mercedes V-Class vans and 5.9% for Sprinter vans.

This is the classic advisory. Your tyres are at, say, 2mm tread depth. Legal minimum is 1.6mm. The tester notes it because in six months you'll fail the MOT, but today you pass. Smart owners replace them immediately because tyres lose grip dramatically below 3mm, especially in wet conditions.

Second most common: shock absorbers with light misting of oil. This appeared in 3.7% of V-Class tests and 1.8% of Tourneo Custom tests. Light misting means the shock absorber seal is starting to weep but hasn't failed yet. It's an early warning, not an immediate problem. Weeping becomes failure when oil is dripping or the shock has lost damping performance.

Third place goes to tyre damage: cuts that don't reach the cords (2.0% of Tourneo tests) and slight perishing or cracking (1.7% of Sprinter tests). These are judgment calls. A small cut in the tread blocks is noted but doesn't fail. A cut in the sidewall approaching the structural layer fails immediately as dangerous.

What's the Actual Difference Between Advisory and Minor?

Here's where it gets confusing. Both advisories and minor defects result in an MOT pass. Both indicate something worth monitoring. So what separates them?

An advisory is purely observational. The tester is saying 'I've noticed this, and you should be aware'. A minor defect is a genuine fault that needs repair, but the fault is so slight it doesn't compromise safety enough to justify a fail.

In practice, minor defects are surprisingly rare in the data. Testers tend to classify most issues as either advisories (if they're borderline) or major defects (if they genuinely need fixing). The minor category exists for things like a slightly loose but not dangerous steering component, or a parking brake that works but needs adjustment.

For buyers, treat advisories and minor defects the same way: budget to fix them within 3-6 months. The mechanical difference might be technical, but the practical advice is identical. Don't buy a car with multiple advisories unless you're prepared to spend money addressing them before the next MOT.

When Does an Advisory Become a Major Defect?

The line between advisory and major defect is often just millimetres or degrees of wear. Understanding where that boundary sits helps you avoid surprises.

Take tyres. At 2mm tread depth, you get an advisory. At 1.6mm (the legal limit), you fail with a major defect. That's a difference of 0.4mm, barely visible to the naked eye. This is why tyre wear advisories should trigger immediate replacement, not a six-month wait.

Brake pads work the same way. When the friction material is approaching the minimum thickness, you get an advisory. When it reaches or goes below that minimum, you fail. The Mercedes V-Class data shows brake pad advisories in 2.3% of tests, a warning sign that these high-mileage vehicles need regular brake inspections.

Shock absorbers demonstrate the progression clearly. Light misting of oil: advisory. Heavy oil leak or reduced damping: major defect. The problem is that 'light' and 'heavy' are subjective. Different testers draw the line in different places, which is why shock absorber advisories are worth investigating rather than ignoring.

How Common Are Dangerous Defects, Really?

Dangerous defects are the category that makes your car immediately illegal to drive. The good news: they're rare on well-maintained vehicles. The bad news: when they appear, they indicate serious neglect.

In our commercial vehicle data, dangerous defect rates ranged from 0.6% (Ford Tourneo Custom) to 1.4% (Mercedes V-Class). These are low figures that reflect the professional maintenance most fleet vehicles receive. Private cars, especially older ones, see higher rates. The DVSA MOT history database shows dangerous defects appearing in roughly 2-3% of all MOT tests nationally.

What counts as dangerous? Complete loss of function or imminent failure. Brake pads worn to the backing plate. Tyres with exposed cords. Suspension components about to separate. Steering with excessive play. Fuel leaks. Severely corroded brake lines. These aren't borderline calls, they're obvious safety failures.

If you're buying a used car and see a dangerous defect in the MOT history, ask hard questions. One dangerous defect suggests an owner who drove until something broke. Multiple dangerous defects across different tests indicate systematic neglect. Walk away.

Why Do Commercial Vehicles Have Fewer Defects?

The vehicles in our dataset, all 2024 model year commercial vans and MPVs, show remarkably clean MOT records. The Ford Tourneo Custom averages just 0.2 defects per test. The Fiat Ducato manages 0.1. Compare this to a typical five-year-old private car, which might average 1.5-2.0 defects per test.

The explanation is simple: business maintenance. Companies running fleets can't afford downtime. They service vehicles on schedule, replace tyres before they reach the limit, and fix small problems before they become MOT failures. The annual mileages tell the story: these vehicles cover 16,000 to 62,000 miles per year. At that rate, missing a service or ignoring wear means expensive breakdowns.

Private buyers should learn from this. The reason these commercial vehicles pass at 96-98% rates isn't because they're better built (though some are). It's because they're better maintained. A seven-year-old ex-fleet van with full service history is often a safer bet than a five-year-old private car with patchy records, even if the van has double the mileage.

The defect data also reveals usage patterns. The Mercedes V-Class shows 17.3% of tests flagging tyre wear, far higher than other vehicles in the sample. This reflects the V-Class role as a luxury passenger carrier: constant motorway miles wear tyres quickly but don't stress other components much. The Sprinter shows more registration plate deterioration (1.5% of tests), suggesting harder commercial use with kerb strikes and loading bay scrapes.

How Should You Use Advisory Information When Buying a Car?

The MOT history is the single most valuable tool for used car buyers, and advisories are the most underused part of that tool. Here's how to read them properly.

First, check the pattern. One or two advisories on an older car is normal. Five or more advisories suggests an owner who does absolute minimum maintenance. Multiple tyre wear advisories across consecutive tests means the owner is stretching tyres to the limit, which tells you how they treat the whole car.

Second, look for escalation. An advisory that appears, disappears, then reappears as a major defect tells you the owner ignored the warning. This is a red flag. You want to see advisories that get fixed, evidenced by them disappearing from the next MOT.

Third, use advisories to negotiate. A car with £400 worth of advisory work pending is worth £400 less. Simple. 'Light misting of oil on shock absorbers' means £300-400 to replace them. 'Tyres worn close to limit' means £200-300 for a new set. These are not minor costs to dismiss.

Fourth, understand timing. An advisory noted in month one of a 12-month MOT certificate is less urgent than one noted in month eleven. If you're buying in November and the MOT expires in December with three advisories listed, factor in an immediate MOT retest cost. Those advisories are now 11 months older.

What Are the Biggest Misconceptions About Advisories?

Misconception one: advisories are just upselling. Some drivers think MOT testers note advisories to generate repair work. While dodgy garages exist, the data shows advisories are legitimate. The same defects appear across different test centres on similar vehicles. Tyres worn close to limit appears in 5-17% of commercial vehicle tests regardless of who's testing. That's real wear, not invented work.

Misconception two: you can ignore advisories until the next MOT. Terrible advice. Components deteriorate between tests. In our data, vehicles averaging 34,000 miles annually are covering nearly 3,000 miles per month. Brake pads and tyres don't wait politely for next year's MOT. They wear continuously.

Misconception three: all advisories are equal. False. 'Slight oil leak from engine, not affecting drip tray' is very different from 'offside front tyre worn close to legal limit'. The first might stable for years. The second becomes illegal within months. Learn to distinguish cosmetic advisories from safety-critical ones.

Misconception four: advisories mean the tester is being cautious. Sometimes yes, sometimes no. Tyre wear advisories are objective: either the tread is approaching 1.6mm or it isn't. Shock absorber misting is more subjective: what one tester calls 'light misting', another might pass without comment. This variance exists, but it doesn't make advisories meaningless. It makes patterns across multiple tests more valuable than individual judgments.

Frequently Asked Questions

Understanding MOT advisories turns the test from a confusing admin task into a practical maintenance guide. The difference between advisory, minor, major and dangerous isn't academic, it's the difference between budgeting repairs sensibly and facing an unexpected £500 bill or buying a car that looked fine but needs immediate work.

When you're checking a car's history, look beyond pass or fail. The advisories tell you how the previous owner treated the vehicle and what you'll be spending in the next 6-12 months. PlateInsight makes this easy by analysing the MOT patterns for any vehicle against 261 million records, showing you whether those advisories are normal for the model or warning signs. Your first five checks are free, giving you the data you need before committing to a purchase.