You can also view previously answered questions in the HSW Question Archive . Gasoline is made up of carbon chains of different lengths ranging from C7H16 through C11H24. If you could burn gasoline as a vapor with a hot flame and plenty of oxygen, you would get nearly pure carbon dioxide and water as the combustion products. That is why you can burn natural gas, LP gas and kerosene indoors in the winter. Automobile exhaust, unfortunately, contains a lot more than carbon dioxide and water.
Carbon monoxide is formed because combustion is incomplete.
Not enough oxygen is available fast enough to react completely with all of the carbon available. Because of the pressure and temperature inside a cylinder, nitrogen and oxygen in the air combine in various ways. There can also be some impurities like sulfur in the gas that form sulfur oxides. The idea behind a catalytic converter is to try to eliminate the carbon monoxide, nitrous oxides and hydrocarbons by reacting them with plenty of oxygen on a platinum catalyst. However, catalytic converters are not perfect, so some of the pollutants escape.