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MOT Failures Guide · Rust & Corrosion

Rust MOT Failures

Structural corrosion is the most expensive category of MOT failure — and the most brutal. A car that was fine last year can fail catastrophically as rust progresses through load-bearing metal. Here is exactly what the MOT checks, what it costs to repair, and how to stay ahead of it.

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Rust MOT Failures

Quick Answer

Surface rust does not fail the MOT. Structural corrosion does. The MOT tester specifically checks chassis rails, subframe mounts, sills, floor pan, brake pipe routes, and suspension mounting points. If any structural area shows perforation under probe testing, it is a failure. Repairs range from £150 for a sill weld to £1,500+ for subframe mounting restoration — often exceeding the vehicle's market value.
The six critical areas

Where the MOT Tester Looks for Structural Corrosion

Dangerous

Chassis rails and longitudinal members

The main backbone structure of the car. Perforation here is a Dangerous failure — the vehicle has lost fundamental structural integrity and could fold in a collision.

Dangerous

Subframe mounting points

Where the front or rear subframe bolts to the main bodyshell. On many older FWD cars, these corrode from the inside. A common failure on Honda Jazz, Vauxhall Astra, and Ford Focus from the early 2000s.

Major/Dangerous

Sill structures

The box sections under the door openings. Sills carry significant structural load, especially in a side impact. Both inner and outer sill are checked. End sections are most vulnerable.

Major

Floor pan

The floor of the passenger cabin. Perforations in the floor pan indicate water ingress and structural weakness. Large holes or heavily corroded sections are failures.

Dangerous

Suspension mounting points

Where suspension components attach to the bodyshell or chassis. Corroded spring towers or wishbone mounts mean the suspension could pull away from the car under load.

Dangerous

Steel brake pipes

Brake lines run along the underside of the car. Heavily corroded pipes are a Dangerous failure — they can rupture during braking. Often not visible from above.

What it costs

Structural Rust Repair Costs UK

Repair typeTypical costDifficultyNotes
Sill weld repair (one sill)£150–£400Specialist weldingMust include sealing and underseal treatment
Full sill replacement (both sides)£800–£2,000Body shopIncludes cut, weld, paint finishing
Subframe mount repair weld£400–£1,500+SpecialistRequires subframe removal on most vehicles
Chassis rail section repair£300–£900SpecialistMust meet structural integrity standard
Floor pan section repair£200–£600Specialist weldingIncludes sealing and sound deadening
Brake pipe replacement (single line)£80–£250SpecialistFull bleed required after replacement
Brake pipe replacement (all lines)£300–£700SpecialistFull brake system bleed essential
Underseal treatment (preventive)£150–£400GarageExtends chassis life significantly — do annually on older cars
Stay ahead

Corrosion Prevention: Keeping Your Car Road-Legal

Annual underseal inspection and topup — especially important after winter salt exposure
Wax-inject sills and box sections every 2–3 years on cars over 10 years old
Address surface rust on panels immediately — treat with rust converter, prime, and paint before it reaches metal depth
Wash the underside annually — pressure wash under wheel arches and along sills to remove road salt
Check drain holes in doors and sills are clear — blocked drains cause water pooling and internal corrosion
Have underside inspected on a ramp every 2 years for vehicles over 8 years old — catch structural rust early
If buying a used car, always check the underside with a torch — structural rust is often hidden from above
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Questions & answers

Frequently Asked Questions

What types of rust fail the MOT?
Surface rust on cosmetic panels (doors, bonnet, boot) does not fail the MOT. Structural corrosion fails the MOT. The tester checks specific structural areas: chassis rails, subframe mounting points, sill end sections, floor pan integrity, suspension mounting points, steering rack mounts, and brake pipe routes. If corrosion has compromised the structural integrity or made a safety component insecure, it is a Major or Dangerous failure.
How is rust assessed at the MOT?
The tester uses a blunt probe (an approved corrosion testing probe) to tap and test suspect areas on a vehicle ramp. If the probe penetrates through the metal under firm pressure, the panel or section is deemed perforated and structurally compromised. The probe cannot be used aggressively — the test is specifically against excessive perforation, not surface oxide.
How much does it cost to repair rust for an MOT?
A sill weld repair: £150–£400 per sill. Chassis rail repair welding: £200–£800. Subframe mounting point repair: £400–£1,500+. Floor pan section repair: £300–£900. Full sill replacement (both sides): £800–£2,000. On severely corroded vehicles, the bill can reach the car's market value — at which point scrapping and replacing the vehicle is more economical.
Can surface rust fail an MOT?
No — surface rust alone does not fail the MOT. Cosmetic rust on wings, door bottoms, wheel arches, and bonnet edges is common on older UK cars and is not directly tested. It becomes an MOT issue only when it has progressed to structural components or has caused perforation in load-bearing areas. However, surface rust is an early warning system — address it before it reaches structural depth.
Does rust on brake pipes fail the MOT?
Yes — heavily corroded brake pipes are one of the most serious rust-related failures, classified as Dangerous. Steel brake pipes on UK cars corrode from the outside in, often becoming so thin that they can rupture under braking pressure. Visually they may look intact while internally compromised. The tester checks for swelling, heavy pitting, and surface perforation.
Which cars are worst for rust MOT failures?
Cars most prone to structural corrosion MOT failures include: Honda Jazz (early 2000s, rear sills), Vauxhall Astra and Zafira (rear subframe mounts, sills), Ford Fiesta (2002–2008, rear suspension mounts), Alfa Romeo 147 and 156 (sills, front subframe), BMW E46 3 Series (rear subframe mounts), and many early 2000s Japanese imports. Salt-belt vehicles, coastal area cars, and those from Scotland and Wales tend to corrode faster.
Check before your test

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SR

Ste Robinson

Founder, MotR · Last reviewed 12 June 2026

Ste Robinson is the founder of MotR, helping UK drivers check MOT history, tax status, vehicle records and ownership information using official DVLA and DVSA data.

Check MOT History for Past Corrosion Advisories

Repeated corrosion advisories on a used car are a major red flag. Check every past advisory free with official DVSA data.

Structural corrosion MOT standards sourced from the DVSA MOT Inspection Manual. Repair costs are UK independent garage / specialist welding estimates, 2026.

Keep reading

MOT Failure Costs UK

Complete repair cost guide covering all MOT failure categories — from £5 bulbs to £1,500 subframe repairs.

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