The “cat” generally refers to the car’s complete exhaust emission control system. The catalytic converter contains an effective catalyst (usually precious metal like platinum or rhodium) on a ceramic or metallic base, a housing, and, depending on the design, various controls to regulate the process. Also depending on its design, the catalytic converter oxidizes carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water, and/or reduces oxides of nitrogen to nitrogen and oxygen gas. Efficient catalytic converters reduce the level of pollutants in the exhaust by as much as 90 percent. Cars with spark-ignition engines use 3-way catalytic converters. Depending on the type of engine, there may be either one or two main converters and up to four pre-converters.