MOT Failures Guide · Exhaust
Exhaust MOT Failures
An exhaust that blows, leaks, or falls apart is an MOT failure. Combined with emissions testing, the exhaust system is one of the most inspected components in the entire test. Here is exactly what fails, why, and what it costs to put right.
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Quick Answer
Exhaust failures cover two separate MOT assessments: the physical condition of the exhaust system (leaks, corrosion, security) and the emissions test (CO, HC, and diesel smoke levels). A blowing exhaust near the passenger compartment is a Dangerous failure. Emissions failures — typically a dead catalytic converter — can cost £300–£1,200 to repair.
Exhaust Failure Types & Repair Costs
| Failure type | Defect severity | Typical repair cost | Notes |
|---|---|---|---|
| Exhaust blowing at manifold joint | Major/Dangerous | £80–£200 | Near passenger cabin — Dangerous classification |
| Exhaust blowing at middle section | Major | £100–£300 | Clamp or weld repair |
| Rear silencer blowing or corroded | Major | £80–£250 | Common on older high-mileage cars |
| Full exhaust system replacement | Major | £200–£800+ | Stainless steel lasts longer — worth the premium |
| Catalytic converter failed/missing | Major + Emissions | £300–£1,200 | Required by law — absence is automatic failure |
| DPF blocked or removed (diesel) | Major + Emissions | £500–£1,500 | Removal since 2014 is an MOT failure by law |
| Lambda/O2 sensor failure | Emissions | £80–£250 | Causes rich running and CO failure |
| Exhaust mounting rubbers broken | Advisory/Major | £20–£60 | Hanging exhaust can become Dangerous |
Pre-MOT Exhaust Check
Listen for ticking, hissing, or rumbling that changes with throttle — exhaust blow symptoms
Look for black soot deposits around any exhaust joints, clamps, or flanges
Check underside with a torch — look for holes, severe corrosion, or sections held by rust alone
Check all mounting rubber hangers are intact and holding the exhaust off the floor
On diesels, check for blue or black smoke under hard acceleration — DPF or fuelling issue
Check the catalytic converter is still present — check conversion is not listed in MOT advisories as removed
Related MOT Failure Guides
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OpenEmissions MOT Failures
What the emissions test measures
OpenRust & Corrosion Failures
How corrosion causes exhaust failures
OpenCommon MOT Failures
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OpenMOT Failure Costs
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OpenMOT History
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OpenMOT Status Check
Current MOT status
OpenMOT Guide
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OpenFrequently Asked Questions
What exhaust problems fail the MOT?
The main exhaust failures are: an exhaust that is blowing or leaking exhaust gases (Major or Dangerous depending on location — particularly dangerous if near the passenger compartment), an exhaust that is so corroded it is in danger of falling off, emissions that exceed legal limits, a catalytic converter that is absent or non-functional, and excessive noise from a damaged or missing silencer box.
How do I know if my exhaust is blowing?
A blowing exhaust typically produces a distinctive ticking, hissing, or rumbling sound that is louder on acceleration and changes character with engine revs. You may also notice the sound changes when you press a cloth against suspect joints while the engine is running. Soot marks or black deposits around exhaust joints or clamps are another clear sign of a leak.
Can I pass an MOT with a blowing exhaust?
A minor blow from a loose clamp that does not affect emissions or create excessive noise may receive an advisory. A significant exhaust blow — especially from the manifold, pre-cat section, or near any heat-sensitive component — will be a Major or Dangerous failure. If a blow is drawing exhaust gases near the passenger compartment, it is an immediate Dangerous failure.
How much does exhaust repair cost in the UK?
Exhaust clamp replacement: £20–£60. Exhaust repair bandage (temporary): £15–£30 DIY. Rear silencer box replacement: £80–£250 fitted. Middle section replacement: £100–£300. Full exhaust system (front to back): £200–£800+. Catalytic converter replacement: £300–£1,200 depending on vehicle. DPF (Diesel Particulate Filter) replacement: £500–£1,500.
What is an exhaust emissions failure?
The MOT measures exhaust gas composition. For petrol engines: carbon monoxide (CO) and hydrocarbon (HC) levels are measured at idle and fast idle. For diesel engines: opacity (smoke density) is measured during a free-rev test. Failures are typically caused by a faulty catalytic converter, failed lambda (oxygen) sensor, incorrect fuel mixture, or a diesel DPF that is blocked or missing.
Can removing the catalytic converter affect my MOT?
Yes — significantly. A catalytic converter is required by law. Its removal will cause the vehicle to fail the exhaust emissions test and may also cause a separate failure for the absence of mandatory equipment. Since 2021 it has been a criminal offence to steal catalytic converters, reflecting their essential function and value.
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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 Full MOT History Including Past Exhaust Failures
Free DVSA data — every advisory, failure, and emissions test result. 30 seconds.
Exhaust and emissions MOT standards sourced from the DVSA MOT Inspection Manual. Repair costs are UK independent garage estimates, 2026.
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Open Emissions MOT Failures
What the MOT emissions test measures, common failure causes, and the difference between petrol and diesel tests.