Not everyone knows or understands the purpose of a catalytic converter. It's meant to convert harmful hydrocarbons, nitrogen oxides and carbon monoxide into less harmful compounds. The catalysts inside the catalytic converter convert carbon monoxide and hydrocarbons into carbon dioxide and water, and nitrogen oxides back into nitrogen and oxygen. With so many vehicles producing extremely harmful exhaust, our oxygen would be depleted without the catalytic converter. A lot of smog and the haziness seen in big cities is caused partly by the toxic fumes emitted from our vehicles.