The oxygen sensor--also known as an O2 sensor--in a 1999 Toyota Corolla determines the amount of oxygen in the Corolla's exhaust system, then sends that data to the engine computer. In turn, the computer determines how much fuel and spark to give to the motor so it will run properly. If the oxygen sensor goes out, you may notice reduced fuel economy or your engine running poorly.
Allow the engine to cool for at least an hour prior to working on the engine. You want to be able to place your hand flat on the top of the engine for at least 10 seconds. If you can do this, then the engine is cool enough to begin working.
Open the hood and locate the oxygen sensor on the front of the exhaust manifold. It is a 3-inch long tube that sticks out perpendicular from the exhaust manifold at the front of the engine between the cylinder head and the radiator, clearly visible when you open the hood. It's also the only thing on the exhaust that has an electrical harness. The wiring harness is approximately 2 inches away from the end of the sensor; it connects directly to the oxygen sensor. Unplug the wiring connection from the sensor to the vehicle using your hands.
Unbolt the sensor from the exhaust manifold, using your hands. Then remove the sensor and the underlying gasket and discard or recycle them.
Place the replacement gasket on the studs, then install the replacement O2 sensor on the exhaust manifold, using the 3/8-inch ratchet and socket. Reconnect the electrical connections on the sensor.
