Found this on google should help.
he oxygen (O2) sensors in a Pontiac Sunfire measure the amount of oxygen in the exhaust gas and relay this information to the powertrain control module (PCM). The PCM uses this information to adjust the air-to-fuel ratio and the ignition timing. There are two O2 sensors in a Pontiac Sunfire: one primary sensor and one secondary sensor. The primary sensor is in the exhaust manifold and the secondary sensor is underneath the car, in the exhaust pipe after the catalytic converter.
Read more :
http://www.ehow.com/how_7325241_o2-s...-sunfire_.html