See Figure 1
Each vehicle is equipped with a decal containing emission control data specific to that vehicle and engine. The decal is typically located on the underside of the hood, but might instead be found on the fan shroud, the coil appearance cover or other engine parts. These specifications are critical to the effective servicing of the applicable system. In addition to the tune-up specifications and procedures, the emission control data decal includes a color-coded schematic of the engine vacuum system. The color coding on the schematic represents the actual color coding on the vacuum hoses, although there may be some exceptions.
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). In order to reduce these forms of pollution, without sacrificing driveability, all vehicles utilize a combination of emission control systems.