An O2 sensor measures the mixture of air to fuel to make sure your vehicle is running at the correct air/fuel ratio. The ideal mixture is 14.7 parts air to 1 part gasoline. Once the oxygen sensor has gathered this information from the exhaust gases, it sends the information to the vehicle's ECU, allowing it to adjust the fuel mix as necessary. This ensures your vehicle will get the best power at the best possible fuel economy. Over time, these sensors do wear out. However, you can check an oxygen sensor before replacing it.
Put the floor jack under the vehicle's front jack point and jack up on it. The front jack point for most vehicles is located behind the radiator and is an extension of the frame of the vehicle.
Place a jack stand under each front pinch weld and lower the vehicle onto the jack stands.
Climb under the vehicle and unplug the electrical wiring from the O2 sensor housing.
Connect the voltmeter leads to the feedback wire and ground terminals on the O2 sensor.
Set the voltmeter dial or option to the millivolt option on your voltmeter.
Start your vehicle. Run it at idle until it warms up to normal operating temperature. Your oxygen sensor will not give you a proper reading unless you are running the vehicle at normal operating temperature.
Check the voltmeter. It should fluctuate and then settle at 150 millivolts. Rev the engine (or have an assistant rev the engine). The voltmeter reading should drop momentarily while the throttle opens. Then, as the throttle closes, the meter should read slightly higher than 150 millivolts. It should then return to the 150-millivolt reading.