The quick answer is: The function of the catalytic converter is to prevent harmful gases from entering the atmosphere therefore decreasing toxic pollution we should not be breathing. The harmful gases exhausted by vehicles are carbon monoxide, hydrocarbons, and nitrogen oxides.
Carbon monoxide is a byproduct from combustion and is considered as a poison when we breathe it and it is extremely toxic and will cause death when inhaled. Nitrogen oxides also a byproduct of combustion is combined with air therefore smog and acid rain is formed by this way which is bad for the environment.
The function of a catalytic converter is to decrease or possibly eliminate emission by a mean of catalyst. It also helps convert harmful gases and substances into friendly gases already naturally occurring in the atmosphere such as carbon dioxide and nitrogen. Inside a catalytic converter are honeycomb like structure so when exhaust gases passes through it; it converts nitrogen oxides to oxygen and nitrogen, hydrocarbons to water and carbon monoxide, and carbon monoxide to carbon dioxide.
One design of a catalytic converter works by a process of oxidation. In other words, it adds oxygen thus changing carbon monoxide into carbon dioxide (a harmless gas). When an oxidation occur, the unburned hydrocarbons released by the vehicle gases turns into carbon dioxide and water, both friendly to the environment. According to studies a vehicle which contains more than one catalytic converter can reduce harmful gases more efficiently compared to than a vehicle having only one catalytic converter.
Catalytic works best when the vehicle is fully warmed up and it make take up to 15 minutes to become effective. During start up, the catalytic converter is less efficient. When temperature reaches over 600 degree Fahrenheit in the catalytic converter, it should work 100%. Anytime a catalytic converter is working less than 95% efficient, a check engine light is triggered to warn driver there is an issue with the vehicle. Normally, a p0420 codes will show when a catalytic converter is no longer working properly. However, be mindful to not replace a catalytic converter based on a p0420 code (learn more about codes and your check engine light here. The catalytic converter should be tested for efficiency and blockage. Use an infrared temperature gun and measure the temperature at the front of the catalytic converter and at the end of the catalytic converter. If the end of the catalytic converter temperature is lower than the front catalytic converter, that means there is a blockage and most likely the catalytic converter is on its way out.