A vehicle’s catalytic converter changes carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NOx) into less harmful pollutants like carbon dioxide (CO2) and water (H20). Nearly all new cars in the US have used them since 1975. Almost every developed country in the 21st century requires new cars to have one.
The catalytic converter is integrated into the car’s exhaust system. In the past, most cars have had underfloor cats, which sit roughly beneath the front seats, on the underside of the car. But more and more use exhaust manifold cats, which connect to the exhaust manifold, and others have maniverters, which form part of the manifold, this allows them to start working quicker, because they warm up faster.
All catalytic converters work in the same basic way. They have a honeycomb core or matrix that is coated with the rare elements rhodium, platinum and palladium. These metals heat up as hot exhaust gases pass over them, and cause the harmful pollutants to chemically react, turning into carbon dioxide, nitrogen and water.
Catalytic converters typically last more than 100k miles without any issue if they are not damaged. Impact damage, from running over a curb or large rock for instance, can cause the matrix inside to fracture and start to deteriorate, eventually breaking down and being exiting via the tailpipe. A faulty oxygen sensor or leaking fuel injector can send too much unburnt fuel into the exhaust which increases the temperature and burns out or melts the matrix. Even something as simple as driving through a deep puddle can damage it from the rapid cooling of the matrix which causes the matrix to break apart.
The catalytic converter works automatically and has no moving parts. Exhaust gasses heat the matrix, and one it is "lit off" it causes the expected chemical reactions in all gasses flowing through it. Most OBDII equipped cars (everything made since 1996) have an oxygen (O2) sensor in the exhaust systems both before and after the catalytic converter in order to monitor it for proper function. If a scan reveals code P0420 - Catalyst System Efficiency Below Threshold it most likely means your cat is no longer performing its job and needs replacement.
How long does a catalytic converter typically last? You should expect a cat to last at least 10 years but most last the life of the vehicle - as long as it isn’t damaged in some way, as mentioned above.