As part of the MOT test most cars are subject to an emissions test. The only cars that are exempt from this rule are vehicles with less than 4 wheels, two-stroke vehicles, hybrid vehicles and quadricycles. Most vehicles are tested using a special meter but old vehicles (classic cars) may be tested by sight. The vehicles are usually linked up to a tube which is placed over the exhaust. The readings are then taken and checked against the MOT standards to say whether it is a pass or fail.
Tests can also be performed on the roadside if needed and if the vehicle fails, you will often get an order to fix the issues within 10 days of face prosecution. You may face a fixed penalty notice on the side of the road in certain counties.
When your car fails on emissions it may be due to an issue with part of the exhaust or the catalytic convertor. The cost of these repairs vary depending on the type of repair needed and then make and model of the car.
If you notice a lot of smoke coming out of your exhaust then you may want to take your car in to a garage before your MOT is due to get them to just test the emissions, this will allow you time to fix the issue before the MOT test and avoid getting a fine or points on your license if you are pulled over.