- bean_bottom_2Lv 51 decade agoFavorite Answer
That is due to incomplete combustion. Automobiles are fueled by petroleum (except a few electric or hybrids). Petroleum is a mixture of carbohydrates. It contains mostly carbon and hydrogen. When there is insufficient oxygen during the combustion process inside a car's engine, carbon will not be complete oxidized into carbon dioxide. Instead, carbon monoxide is formed.