An O2 sensor measures the contents of the exhaust gasses coming out of the engine. These types of sensors are typically plugged into the catalytic converter or exhaust system to measure how many toxic materials are being emitted from the engine's combustion chambers. While this may be useful for emissions control, it can hinder performance of the vehicle if the sensor fails. After roughly 100,000 miles, if your engine has trouble idling or starts misfiring, it may be due to a bad oxygen sensor. However, you should troubleshoot the problem first before replacing anything.
Raise your vehicle onto jack stands if necessary. Jack up on the front jack point located behind the radiator.
Unplug the electrical connection to the O2 sensor. The upstream oxygen sensor is typically found on the exhaust manifold that is bolted to the engine block. It may be as far up on the exhaust as the the manifold piping located in the front of the engine under the hood. The sensor looks like a plug sticking out of the exhaust piping and will be the only plug inserted into the exhaust system.
Connect the voltmeter to the feedback wire and ground on the O2 sensor. The feedback wire lead and ground lead will be visible once you unplug the factory harness from the sensor. The leads are what the factory harness connects to. There will be two leads. One will be the feedback wire, the other will be the ground. The exact orientation of the leads depends on your vehicle. You will need to consult the particular vehicle's manual.
Turn the dial on the voltmeter to "millivolt".
Start your engine and let it warm up to normal operating temperature.
Note the voltage. It should settle at roughly 150 millivolts after the initial startup. Rev the engine and check the voltmeter (you may need an assistant for this). The voltmeter reading should drop momentarily as the throttle opens, and then it should read slightly higher than 150 millivolts as the throttle returns to the idle position. It should eventually settle back to 150 millivolts. If this does not happen, then the O2 sensor is bad and needs to be replaced.
