Will my emissions fail my MOT?

Wednesday 5th August 2026

Yes, emissions can fail your MOT if they exceed the legal limits for your vehicle. An emissions fail is always a major fault, so you cannot legally drive the car away until it passes a retest, except to a pre-booked repair. If your vehicle is emitting thick black smoke it will be classified as dangerous and cannot be driven at all. It will need to be towed if it cannot be repaired in situ. Most MOT test fails caused by emissions come from a handful of common, fixable causes, and a few checks before your test can reduce the risk of failing.

This guide explains how testers check emissions, why vehicles fail and how you can lower your emissions before your MOT.

How are emissions tested?

The emissions check applies to every petrol vehicle first used on or after 1 August 1975, and every diesel vehicle first used on or after 1 January 1980. It has formed part of the MOT since 1991 for petrol cars and 1994 for diesel cars.

Petrol cars are checked for carbon monoxide, hydrocarbons and the lambda value (a measure of how precisely the engine burns fuel). Diesel cars are checked for smoke opacity (how much the exhaust smoke blocks light due to the density of soot particles present). Hybrids don’t need these tests.

What happens in an emissions test?

Petrol cars

The tester inserts a probe into the tailpipe while the engine runs at a fast idle of 2,500–3,000 rpm, then again at a normal idle. The analyser records the carbon monoxide, hydrocarbons and, on cars fitted with a catalytic converter, the lambda value.

To pass, the readings must meet all of the following:
Carbon monoxide: no more than 0.2% at fast idle
Hydrocarbons: no more than 200ppm at fast idle
Lambda: between 0.97 and 1.03 at fast idle
Carbon monoxide: no more than 0.3% at normal idle

Older vehicles without a catalytic converter are tested at idle only, against more generous limits: carbon monoxide up to 3.5% and hydrocarbons up to 1,200ppm.

Diesel cars

Diesel cars are tested for smoke opacity using an approved smoke meter. The tester revs the engine to clear the system, then presses the accelerator quickly to full throttle and reads the smoke density. The maximum smoke level depends on the figure shown on the vehicle's manufacturer's plate, or a default limit where no plate value is shown:

2.5m?¹ for a non-turbocharged engine first used before 1 July 2008
3.0m?¹ for a turbocharged engine first used before 1 July 2008
1.5m?¹ for vehicles first used between 1 July 2008 and 31 December 2013
0.7m?¹ for vehicles first used on or after 1 January 2014

Any vehicle fitted with a diesel particulate filter (DPF) also fails if it emits visible smoke of any colour during the test, even if the metered reading is within limits.

Visual and equipment checks

Both fuel types also get a visual check of the emissions control equipment, including the catalytic converter, DPF, oxygen sensors and EGR valve (a component that reduces emissions by recirculating some exhaust gas back into the engine). Missing, modified or faulty equipment is a major fault regardless of the gas or smoke reading. The tester also checks the engine management light. If it stays on, the car fails, whatever the other results show.

Why do emissions cause MOT test fails?

Most emissions test fails trace back to one of these causes:

Faulty oxygen or lambda sensor: A worn or failed sensor sends incorrect data to the engine control unit, causing the engine to run rich. Carbon monoxide and hydrocarbon readings then climb above the limit.
Failing catalytic converter: Catalytic converters degrade with age and contamination. A converter that no longer converts exhaust gases efficiently pushes carbon monoxide and hydrocarbons past the threshold.
Blocked or faulty DPF: Frequent short journeys prevent a DPF from regenerating (burning off trapped soot) properly, letting soot build up. A blocked filter causes visible smoke or exceeds the opacity limit.
Worn engine components: Worn piston rings, valve seals or injectors let oil or excess fuel into the combustion process, increasing both smoke and gas emissions.
Exhaust or induction leaks: A leak near the sensor lets unmeasured air into the sample, skewing the reading and causing an automatic major fault.
Old or contaminated engine oil: This affects combustion efficiency and can push borderline readings over the limit

How can you lower emissions before your test?

Here at Formula One Autocentres, we recommend getting a full service before your MOT test. We even offer a discount when you book both together.

A service picks up worn spark plugs, dirty air filters and old oil, all of which affect combustion and emissions.
If your engine management light is on, get it diagnosed and fixed before your test. Don't wait until test day.
Before a diesel test, take the car on a longer motorway drive. A 20–30 minute run a few days beforehand can burn off soot trapped in the filter.
Check the exhaust visually for leaks or excessive smoke.