Road vehicles are an air pollutants and it is the role of the catalytic converter to minimise the impact by converting such pollutants into less harmful emissions before they leave the exhaust.
A catalytic converter can last the lifetime of the vehicle but sometimes they burn out, get clogged up or get damaged from a collision, Most modern cars will make you aware of a faulty catalytic converter by triggering warning light. Other telltale signs are loss of power to the engine, hesitation when accelerating or going up inclines, drop in fuel economy or strange odours.
A faulty catalytic converter would lead to an MOT failure to due high emissions so it is important to get it checked out and replaced as soon as possible. The catalytic converter can be easily replaced at low cost with moderate mechanical knowledge.