Emissions MOT failures: diesel smoke, DPFs and catalytic converters
Emissions failures are less common than lights or tyres, but they're among the most expensive – and the most likely to make an older car uneconomic.
Checked against the MOT inspection manual and GOV.UK guidance · Updated November 2026
Petrol cars
For petrol cars with a catalytic converter, the tester uses a four-gas analyser in the exhaust after the engine is warmed. It measures:
- Carbon monoxide (CO) at a fast idle and at a normal idle
- Hydrocarbons (HC) at fast idle
- Lambda – the air/fuel ratio, which must sit within a narrow band around 1.0 (typically 0.97–1.03)
Limits depend on the car's age and the manufacturer's specific figures where available. For most modern petrol cars the default limits are very tight – around 0.2–0.3% CO and 200 parts per million HC. A healthy catalytic converter easily beats them; a failing one won't come close.
The tester also checks the engine management light. An illuminated malfunction indicator lamp is a fail in itself.
Diesel cars
Diesels are tested with a smoke opacity meter. The engine is revved from idle to its governed speed several times and the meter measures how much light the smoke blocks. The limit is the manufacturer's "plate value" on a sticker under the bonnet or in the handbook; where there's no plate value, a default applies depending on age – as low as 1.5m-1 for cars first used from July 2008.
Since May 2018 two extra rules apply to diesels with a diesel particulate filter (DPF):
- Any visible smoke of any colour during the test is a major fail.
- Evidence that the DPF has been removed or tampered with is a major fail, unless it's clearly been removed temporarily for cleaning.
Common emissions failures
| Defect found | Usual category |
|---|---|
| Petrol CO, HC or lambda outside limits | Major |
| Diesel smoke opacity over limit | Major |
| Visible smoke from a DPF-equipped diesel | Major |
| DPF removed or evidently tampered with | Major |
| Catalytic converter missing where originally fitted | Major |
| Engine malfunction indicator lamp on | Major |
| Emission control equipment missing, modified or obviously defective | Major |
What causes emissions failures
Petrol
- A failing or contaminated catalytic converter – often after a misfire has sent unburned fuel through it
- Faulty lambda (oxygen) sensors
- Misfires from worn spark plugs, coil packs or injectors
- Vacuum leaks, a failed coolant temperature sensor, or a car simply not warmed up
Diesel
- A blocked DPF – very common on diesels used for short journeys, which never get hot enough to regenerate
- A sticking EGR valve
- Worn injectors, a faulty turbo, or a dirty air filter
- Cam belt tension and engine wear on high-mileage diesels
Repair costs
| Repair | Typical cost | DIY? |
|---|---|---|
| Diagnostic scan | £40–£90 | Code reader for basics |
| Lambda sensor | £80–£250 | Often |
| Spark plugs and/or coil pack | £80–£300 | Often |
| EGR valve clean / replace | £150–£600 | Sometimes |
| DPF forced regeneration / chemical clean | £100–£300 | No |
| DPF replacement | £600–£2,000+ | No |
| Catalytic converter (aftermarket, fitted) | £300–£900 | Sometimes |
| Catalytic converter (OE, fitted) | £800–£2,500+ | No |
| Injector replacement (each, diesel) | £200–£500 | No |
Typical UK independent garage prices including parts, labour and VAT, 2026. Main dealers and premium/4x4 vehicles cost more.