Many cars are fitted with a catalytic convertor. The job of a cat is to substantially reduce the amount of harmful pollutants produced by the car engine by taking these gases and converting them into water vapour and less harmful gases. This is then expelled through the exhaust system.
Over time, cars that are fitted with a catalytic convertor can start to develop problems and this is usually when it becomes clogged up with unspent fuel and residue. This build up on the inside of the catalytic converter prevents the catalyst from removing the harmful emissions from the exhaust. If you have recently had an MOT and your emissions are high, this could be the reason.
Catalytic converter cleaner removes the carbon build up in your engine and exhaust system. It may often need to be used up to four times a year to see the results as if the cat has been clogged up for some time then one application may only get rid of a fraction of it. Make sure you always read the bottle to see how much fluid you need to put in and how long you should run the vehicle afterwards to ensure that it has gone through the system.