What is the purpose of the catalytic converter in the exhaust system?

The catalytic converter is part of the exhaust system that reduces harmful emissions. It uses precious-metal catalysts to convert toxic gases — carbon monoxide, unburned hydrocarbons, and nitrogen oxides — into less harmful substances like carbon dioxide, water vapor, and nitrogen before they leave the tailpipe. Catalytic converters are required for emissions control on modern gasoline vehicles. A failing converter can trigger a check-engine light and cause performance problems. Understanding the catalytic converter's role in cleaning exhaust is part of the fuel-and-emissions content on the Auto subtest.