In 1970 Congress passed the Federal Clean Air Act, which set quality standards for the nation. The act required areas that did not meet air quality standards to implement emissions control strategies or they would lose their building and highway funds. California did not meet the Federal Air quality standards. Hence, in 1982 it decided to implement an emissions inspection program, which is currently referred to as the" Smog Check" program. I'm not sure how this works in other states, because I heard from my boyfriend in Kentucky that he doesn't need to undergo a smog check every 2 years like California residents with cars newer than 1973 (that don't burn diesel, etc). Yet residents of a different county in the same state do have to pass a smog check. ::head scratching ensues::

The Smog Check program is designed to identify vehicles, which are polluters through the use of an exhaust gas analyzer. The exhaust analyzer is much like a photo camera except it takes pictures of your vehicle's exhaust and translates that into information which a smog technician can understand. Based on this information the technician can diagnose your vehicle's engine condition.

The Visual Inspection

During the visual portion of the smog inspection, the smog technician will be looking for the presence and proper connection of several different emission components, generally those that are designed to reduce overall engine emissions. The technician visually locates and verifies that the components are all properly connected, looking for defects and disconnections, etc. The following is a list of emission components that will be examined:

Exhaust Gas Recirculation Valve (EGR)
Positive Crankcase Ventilation valve (PCV)
Charcoal Canister (Evaporation Control System)
Catalytic Converter (CAT)
Oxygen Sensor (O2 Sensor)
Air Injection System (AIR)
Pre-Heat Tube

The Functional Inspection

The functional inspection is also performed by the smog technician and includes examination of your vehicle's:

"Check Engine" light
Gas cap
Ignition timing (if applicable)
Exhaust recirculation valve (EGR valve, if applicable)

Emissions Inspection

This is the third (and most often failed section) of the smog test. To pass this inspection your vehicle's engine must be running fairly well, insuring all fuels in its chambers are burning away efficiently and cleanly. Consequently this is the entire reason for the smog inspection program.

Smog is formed due to improperly burned fuels. In cases where too much fuel is presented to the engine cambers high levels of carbon monoxides (CO) are emitted through the engine tail pipe. In cases where low fuel levels are presented, once again the fuel burns improperly and now the engine emits high levels of nitrous oxides (NOx). The engine may also miss-fire due to inadequate fuel, creating excessive hydrocarbons (HC).

The optimum performance level is achieved when the engine can maintain a 14.7:1 air/fuel ratio during the combustion process with a constant variation +/- 5% to assist the catalytic converter operation.