Exhaust, noise and fluid leak MOT failures
The MOT section called 'noise, emissions and leaks' covers the exhaust system, the fuel system and any fluid leaking from the car.
Checked against the MOT inspection manual and GOV.UK guidance · Updated November 2026
Exhaust system
The tester checks the whole exhaust from the manifold to the tailpipe for leaks, security, damage and corrosion. A blowing exhaust is easy to hear – especially at idle under the car – and is a major defect. Leaks before the oxygen sensors can also cause an emissions fail by letting air in.
- Exhaust leaking or insecure
- Silencer missing or obviously defective (excessive noise)
- Exhaust mounts or hangers broken
- Exhaust fumes entering the cabin
Excessive noise
The MOT doesn't measure decibels for cars, but the tester judges whether the exhaust is much louder than a standard one of that type. Aftermarket exhausts are fine if they're in good condition and not excessively loud; a "decat" or straight-through pipe is not.
Fluid leaks
Any fluid leak is checked for whether it's likely to harm the environment or create a fire or safety risk:
| Defect found | Usual category |
|---|---|
| Exhaust leaking or insecure | Major |
| Silencer missing or obviously defective | Major |
| Oil leaking excessively (continuous drip) | Major |
| Fluid leak likely to cause fire, or posing a serious environmental risk | Dangerous |
| Fuel leaking, or fuel filler cap missing or not sealing | Major / Dangerous |
| Fuel tank or pipes insecure or damaged | Major |
| Coolant or other fluid leaking excessively | Major |
A slight oil weep is normally an advisory or minor. A continuous drip – the tester typically watches for drops forming – is a major. A fuel leak is treated as dangerous because of the fire risk.
Fuel filler cap
An easy one: the fuel cap must be present and seal properly. A missing cap is on the free retest list.
Repair costs
| Repair | Typical cost | DIY? |
|---|---|---|
| Exhaust back box | £80–£250 | Often |
| Centre section / mid pipe | £80–£250 | Often |
| Exhaust flexi joint repair | £60–£150 | Sometimes |
| Exhaust manifold gasket | £100–£300 | Sometimes – seized studs |
| Full exhaust system (non-performance) | £300–£800 | Sometimes |
| Rocker/cam cover gasket (oil leak) | £80–£200 | Often |
| Sump gasket or seal | £100–£350 | Sometimes |
| Fuel pipe or hose | £80–£300 | Specialist |
| Fuel filler cap | £10–£40 | Yes |
Typical UK independent garage prices including parts, labour and VAT, 2026. Main dealers and premium/4x4 vehicles cost more.