Here are the major components off the EPA site.
http://www.epa.gov/OTAQ/consumer/warr95fs.txt
What Are Specified Major Emission Control Components?
There are three specified major emission control components,
covered for the first 8 years or 80,000 miles of vehicle use on 1995
and newer vehicles:
* Catalytic converters.
* The electronic emissions control unit or computer (ECU).
* The onboard emissions diagnostic device or computer (OBD).
Catalytic converters are critical emission control components
that have been installed on most cars and trucks manufactured since
1975. Since engines don't burn fuel completely during the combustion
process, the exhaust contains a significant amount of harmful
pollutants such as carbon monoxide, hydrocarbons, and oxides of
nitrogen. The catalytic converter aids the conversion of these
pollutants to less harmful substances such as carbon dioxide, water
vapor, nitrogen, and oxygen before the exhaust is expelled into the
environment.
The electronic emissions control unit or computer monitors
certain powertrain functions and controls various operating parameters
to help the vehicle run efficiently and with the lowest possible
emissions. Ignition, transmission function, air injection, exhaust
gas recirculation (EGR), engine operating temperature and fuel system
parameters are some of the systems monitored and/or controlled by the
electronic emissions control unit.
The onboard emissions diagnostic device monitors the operation of
a vehicle's emission control system and alerts the driver with a
dashboard light when malfunctions occur. The system will record where
the problem is occurring and assist automotive technicians in
diagnosing and repairing emission control malfunctions. Since some
emission control malfunctions do not have an adverse effect on vehicle
performance, they can go undetected by the driver for quite some time.
The onboard diagnostic device will help catch malfunctions early,
preventing a significant output of harmful exhaust emissions from your
vehicle, and possibly in time to be covered by the emissions control
warranty. Often this "device" is part of the electronic control unit
mentioned above.
