See Figure 1
There are three basic sources of automotive pollution in the modern internal combustion engine. They are the crankcase with its accompanying blow-by vapors, the fuel system with its evaporation of unburned gasoline, and the combustion chambers with their resulting exhaust emissions. Pollution arising from the incomplete combustion of fuel generally falls into three categories: hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).
All vehicles (covered in this guide) have an Emissions Control Information Decal, mounted on the radiator support or possibly on the air cleaner housing. Always refer to this decal when performing service to the emission controls.