Why Do MOT Tests Include an Exhaust Emission Test?

An exhaust emission test measures the amount of pollutants your vehicle emits through its exhaust system. Specifically, it measures carbon monoxide (CO) and hydrocarbons (HC), alongside things like air-to-fuel ratio and engine/fuel efficiency. If you have a diesel vehicle, the test will also measure particulate matter (PM) and smoke opacity.

Basically, the test is measuring how much your vehicle contributes to air pollution and other aspects that pose risks to public and environmental health.

To carry out the test, technicians will insert a probe into your vehicle's exhaust pipe. The probe is connected to a gas analyser, which measures the concentration of pollutants in your vehicle's exhaust. Once the technician has enough readings, they will compare the results against legal limits.

The legal emission limits for UK vehicles aren't universal. They vary depending on factors like the age, fuel type, and engine specifications of the vehicle. However, the ultimate goal is the same no matter what you're driving: to make sure that emissions stay within acceptable environmental thresholds.

Emission tests are a legal requirement for MOT tests under UK law. The general aim is to reduce the environmental impact of motor vehicles by identifying high-polluting engines before they can contribute too much to global warming and poor air quality.

Why is this so important? Well, exhaust pollutants are a major risk factor for both environmental and human health. Air pollution is a leading cause of respiratory and cardiovascular diseases, especially in built-up areas. Gases and particles from vehicle exhausts can exacerbate conditions like asthma and bronchitis.

Of course, it's not just public health at stake. The environment is also a huge factor in regulating and reducing vehicle emissions. The UK aims to be Net Zero by 2050, and is introducing Clean Air Zones in a number of major cities. The success of these goals depends on slashing greenhouse gases, which in turn depends on monitoring and regulating your car's emissions.

Exhaust emission tests are relatively simple procedures. Here's a step-by-step look at the process:

1. Engine warm-up: The technician will turn on your engine and run it until it reaches normal operating temperature.
2. Probe inserted into the exhaust: The technician inserts a sampling probe into the exhaust pipe to check pollution produced.
3. Emissions measured and compared to limits: The analyser measures the levels of pollutants and compares them with the legal limits for that vehicle type.
4. On-board diagnostics (OBD): For newer vehicles, the MOT may also include an electronic diagnostic check, which looks for things like malfunctioning sensors or error codes stored in the car’s computer.

Emission standards vary depending on things like the age, fuel type, and sometimes engine size. Vehicles are tested against the standard in place at the time they were first registered. As a result, older vehicles get a degree of leniency while newer vehicles must meet stricter thresholds, especially those fitted with catalytic converters or DPFs.

You can find the full details of MOT emissions standards here. However, standard limits for petrol and diesel vehicles are as follows:

Petrol vehicles

The basic emission test for petrol vehicles is carried out on all petrol vehicles first used on or after 1st August 1992. It involves testing CO and HC levels with a further check made with a lambda sensor, which measures how effectively the engine controls its air-to-fuel ratio.

To start with, the engine is run at a speed of 2500–3000 rpm (fast-idle speed), during which the CO emissions must be less than 0.2%. HC should be at or less than 200 parts per million and the lambda must be between 0.97 and 1.03. Then, the engine will be run at 450–1500 rpm. This is classed as ‘normal idle speed’ and the CO levels must be below 0.3%.

If the vehicle passes both tests, it meets the emissions standards. However, if it fails, the tester will confirm if the car was originally fitted with a catalytic convertor. Cars that aren’t supposed to have a catalytic converter may be retested using more relaxed limits. Whereas, those that should meet modern standards will be automatically subjected to a more detailed emission test. 

Diesel vehicles

Diesel vehicles are primarily tested for the density of smoke emissions, and (for newer models) how effective systems like the Diesel Particulate Filter (DPF) are. To do this, the tester will conduct a metered smoke test. 

First, the engine speed will be run at around 2500 rpm to check it’s working correctly, then gradually increased to maximum speed. This will check the operation of the fuel pump governor.

If the engine stabilises at its maximum speed, it will be returned to idle speed, ready for the smoke test. This involves inserting a sampling probe into the exhaust pipe. If it doesn’t stabilise, it will fail the test. 

If your vehicle fails its MOT emission test, it cannot be legally driven except to a pre-booked appointment at a garage or test centre.

Common causes of failure include:

• Faulty oxygen sensors: These monitor the air-fuel mixture and send signals to the Engine Control Unit (ECU). A malfunction can make fuel efficiency lower and cause high emissions.
• Dirty air filter: A clogged air filter can restrict airflow to the engine. This can reduce combustion efficiency and increase emissions.
• Worn catalytic converter: The catalytic converter turns harmful gases into less harmful ones. As it wears out, it becomes less efficient, allowing more pollutants into the exhaust.
• DPF issues (for diesel vehicles fitted with DPFs): Problems with the Diesel Particulate Filter often lead to instant failure.

If your vehicle fails, the test centre will issue a VT30 'refusal of an MOT test' certificate. This means that your vehicle will fail its MOT and will not be legally roadworthy. It will be up to you to get your vehicle repaired and retested. 

There are several ways that you can give your vehicle the best possible chance of passing its exhaust emission test:

• Book regular servicing: Well-maintained engines are more efficient and produce fewer emissions.
• Use quality fuel: Cheap or poor-quality fuel can increase carbon build-up in the engine.
• Drive your car before the MOT: Warming up the engine and getting it to optimal temperature can help reduce cold-start emissions.
• Check the warning lights: If the engine management light is on, have it checked before the MOT.
• Replace air filters and spark plugs: These parts are easy to overlook, but they can significantly impact combustion efficiency.
• Avoid short journeys: Frequent short trips can clog up filters and prevent your vehicle's engine from running efficiently.