The Pontiac Sunfire uses two heated oxygen sensors. Basically, the car has an internal heater to help it reach operating temperature and to communicate via voltage to the engine management computer. If the O2 sensor goes bad or fails altogether, it can cause damage to other exhaust components such as the catalytic converter.
Make sure the oxygen sensor does need to be replaced. Check the engine code with a OBDII code reader, or have it tested for free at an auto parts store.
Sensor 1 can be replaced by raising the hood. You'll have to go under the car to replace Sensor 2. Lift the vehicle with the floor jack and place the jack stands under it. Put on the safety glasses, and crawl under on the creeper with other tools.
Locate the oxygen sensors. Sensor 1 is in front of the engine compartment in the middle located on the manifold. Sensor 2 is under the vehicle behind the catalytic converter.
Unplug the wire of the oxygen sensor from the wire harness plug connection.
Place the oxygen sensor socket onto the ratchet, and remove the oxygen sensor.
Install the new oxygen sensor and tighten. Plug the oxygen sensor wire back into the wire harness.
