The majority of cars have one catalytic converter. However, this is dependent on the number of exhaust pipes the vehicle has. Generally, each car has one catalytic converter per exhaust tube; if a vehicle has a dual exhaust system, it has two catalytic converters.
Catalytic converters first appeared on cars in 1975, after legislation limiting exhaust emissions came into force. The function of a catalytic converter is to convert harmful pollutants into less-harmful ones before the gases exit a car's exhaust system.
The main exhaust gases from a combustion engine are water vapor, carbon dioxide and nitrogen. While water vapor and nitrogen are benign, carbon dioxide has links to global warming. Due to inefficiencies in the combustion process, other more harmful gases are also produced in small amounts. Nitrogen oxides are major causal factors of smog and acid rain; carbon monoxide is odorless, colorless and poisonous; and hydrocarbons and other volatile organic compounds resulting from unburned fuel are further major contributors to smog.
Most modern vehicles have three-way catalytic converters, which are used to reduce these three principal pollutants resulting from the combustion process.
The catalytic converter itself is a two-stage process: reduction and oxidation. The initial reduction catalyst acts to decrease nitrogen oxide emissions, while the second-stage oxidation catalyst acts to decrease hydrocarbons and carbon monoxide.