Every street legal vehicle manufactured in the past several decades has at least one catalytic converter mounted within its exhaust system. Many luxury cars have two, or even more. The purpose of the catalytic converter is to superheat unburned particles in your engine's exhaust gases. As your engine warms up, the catalytic converter will heat to an operating temperature of 500 - 1200°F. This allows it to burn harmful particles in the exhaust gases, converting them to water vapor and carbon dioxide. The catalyst inside is usually made of precious metals such as platinum, rhodium, or palladium, which means that catalytic converters usually cost several hundred dollars. Thieves sometimes target catalytic converters because of this, jacking cars up and cutting them out with hacksaws. When your vehicle's catalytic converter is no longer able to burn off the unburned hydrocarbons in the exhaust, your exhaust gases may begin to smell abnormal, your engine may run rough or misfire, and your check engine light will most likely come on.
