The 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.
Turn the ignition to the "OFF" position and shift the Sunfire's transmission into "Park" or first gear (manual). Set the parking brake and open the hood. Raise the front of the vehicle using an automotive jack and support it with jack stands. Chock the rear wheels.
Locate the exhaust manifold. It is inside the engine compartment, made from cast iron and bolted to the front of the engine. The primary oxygen sensor is located on the bottom of the exhaust manifold, and has a single wire connector attached to it.
Locate the catalytic converter. It is underneath the vehicle, oblong and in the center of the exhaust pipe. The secondary oxygen sensor is located six inches downstream from the catalytic converter. It has a single wire connector and screws into the exhaust pipe.
