It means the tester saw rust that has not yet weakened a structural area, so the car passed. If corrosion spreads into a prescribed area near load-bearing parts, it becomes a major or dangerous failure.
Last reviewed: June 2026
What a corrosion advisory means
Rust is an almost unavoidable part of owning an older car on UK roads, where damp weather and winter road salt accelerate it. When an MOT certificate notes corrosion as an advisory, the tester has seen rust on the vehicle but judged that it has not yet reached the point of compromising a structural area. The car has passed, and the advisory is a record that the rust is present and worth watching. It is one of the most common advisories on vehicles more than a few years old.
- A corrosion advisory means rust was seen but it has not yet weakened a structural or load-bearing area enough to fail.
- DVSA assesses corrosion against the MOT inspection manual, which defines prescribed areas around load-bearing components and mountings.
- Corrosion within a prescribed area that affects structural strength is a major or dangerous defect and an MOT failure.
- The dangerous, major, minor and advisory defect categories were introduced in May 2018.
- Driving without a valid MOT can bring a fine of up to £1,000 under the Road Traffic Act 1988.
The key to understanding the advisory is that not all rust is treated equally. The MOT is concerned with corrosion that affects safety, principally the strength of the structure and the security of components such as brakes, steering and suspension that bolt to it. Light surface rust on a panel, a bracket or an exposed bolt is cosmetic and will usually be noted, if at all, as an advisory. Corrosion that eats into the metal that holds the car together is a different matter and is assessed far more strictly.
Surface rust versus structural corrosion
Surface or cosmetic corrosion is the familiar orange staining that appears on exposed steel. It sits on the surface, has not significantly reduced the thickness of the metal, and does not affect how strong a component is. Examples include light rust on the underside of a panel, on a subframe surface or on suspension brackets that remain solid when checked. This kind of corrosion is unsightly and will worsen if ignored, but on its own it does not fail an MOT.
Structural corrosion is corrosion that has weakened metal which the car relies on for strength. This includes the chassis, sills, floorpan, suspension and subframe mountings, and the areas around seatbelt anchorages and brake and steering components. When rust in these places has reduced the metal so that it flexes, flakes away or can be deformed under pressure, it is no longer cosmetic. A tester checking these areas may tap or apply firm hand pressure to suspect metal: if it gives way or the corrosion is excessive, the vehicle fails. The judgement is about whether the structure can still do its job safely.
How testers assess corrosion in prescribed areas
The MOT inspection manual defines what it calls prescribed areas. These are zones within a set distance of load-bearing components and their mountings, such as the points where suspension arms, subframes, steering racks and seatbelts attach to the body or chassis. Corrosion is assessed more strictly inside a prescribed area because failure of metal close to these mountings could allow a critical component to come loose. The principle is that the nearer corrosion is to a mounting that carries load, the more serious it is treated.
Within a prescribed area, a tester examines the condition of the metal and judges whether the corrosion has significantly reduced its strength. Corrosion that is excessive, or that has weakened the structure within a prescribed area, is recorded as a major defect and fails the test. Where the deterioration is severe enough that a component is insecure or the structure could fail in use, it is classed as a dangerous defect, which means the vehicle should not be driven until repaired. Outside a prescribed area, the same amount of surface rust may only warrant an advisory. The table below summarises how location and severity combine to set the category.
| Corrosion type and location | MOT category | Outcome |
|---|---|---|
| Surface rust on a panel, not structural | Advisory | Pass, monitor and treat |
| Light corrosion near but not weakening a mounting | Advisory or minor | Pass, watch closely |
| Excessive corrosion in a prescribed area | Major | Fail, repair required |
| Corrosion making a component insecure | Dangerous | Fail, do not drive |
Can you keep driving with a corrosion advisory
Because an advisory is recorded on a vehicle that has passed, the MOT remains valid and the car is legal to drive for the full period. The advisory does not impose a deadline. What it does is warn that rust is present and will continue to develop, often more quickly underneath where it is hidden from view. Corrosion that is cosmetic at one test can spread into a structural area by the next, turning a pass into a failure.
The sensible response is to inspect the affected area before the next test rather than waiting for it. Having the underside checked and the corrosion treated while it is still cosmetic is far cheaper than welding repairs to a structural section, which can be costly and on older cars may exceed the value of the vehicle. An advisory is the point at which intervention is cheapest and most effective.
How to slow corrosion before the next test
Corrosion is driven by moisture, oxygen and salt, so reducing exposure to these slows it down. Washing the underside of the car periodically, especially after winter when roads have been salted, removes the salt that accelerates rust. Keeping the vehicle dry and ventilated, and clearing leaves and debris from drainage channels and sills where water collects, all help.
Where surface rust has already formed, treating it before it spreads is the most effective measure. This typically involves removing the loose rust, applying a rust converter or primer and protecting the area, and applying underbody or cavity protection to vulnerable sections. Work on structural areas, and any welding, should be carried out by a competent professional, because a poor repair near a load-bearing mounting can itself be unsafe and may not pass the next test. Slowing corrosion early is far easier than reversing structural damage later.
The vehicle MOT history available online records advisories from past tests, so a recurring corrosion note across several years is a useful signal that an area is steadily deteriorating and may be approaching a failure. Owners buying an older car can check this history before purchase to gauge how far rust has progressed. For a car that is kept long term, treating corrosion as soon as it is first flagged, and repeating underbody protection every year or two, is the most reliable way to keep a vehicle through many more MOT cycles without expensive structural work.
Frequently Asked Questions
What does an MOT corrosion advisory mean?
It means the tester found rust on the vehicle that has not yet weakened a structural or load-bearing area enough to fail. The car passed, and the advisory is a record that corrosion is present and should be monitored and treated before it spreads into a more serious area.
When does corrosion fail an MOT?
Corrosion fails when it has significantly weakened the structure within a prescribed area around load-bearing components or mountings, or where it makes a component insecure. Such cases are recorded as major or dangerous defects. Cosmetic surface rust that does not affect strength does not cause a failure.
What is a prescribed area for MOT corrosion?
A prescribed area is a zone within a set distance of load-bearing components and their mountings, such as suspension, steering, subframe and seatbelt attachment points. Corrosion within these zones is assessed more strictly because weakened metal near a mounting could allow a critical component to come loose.
Can I drive with a corrosion advisory?
Yes. An advisory is recorded on a vehicle that has passed, so the MOT remains valid and the car is legal to drive. The advisory is a warning that rust is present and progressing, so the affected area should be inspected and treated before it develops into a failure.
How do I stop corrosion getting worse before the next MOT?
Washing the underside to remove road salt, keeping the car dry and clearing blocked drainage points all slow corrosion. Treating existing surface rust with a converter or primer and applying underbody protection helps further. Structural repairs and welding should be left to a competent professional.
