There are three sources of automotive pollutants; crankcase fumes, exhaust gases, and gasoline evaporation. The pollutants formed from these substances fall into three categories: unburned hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The equipment used to limit these pollutants is called the emission control system.
Due to varying state, federal, and provincial regulations, various specific emission control components have been devised for each. The U.S. emission equipment for vehicles covered by this manual is divided into two categories: California and 49 State (Federal). In this section, the term "California'' applies only to trucks originally built to be sold in California. California emissions equipment is generally not shared with equipment installed on trucks built to be sold in the other 49 States. Models built to be sold in Canada also have specific emissions equipment, although in most years 49 State and Canadian equipment is the same.