How to Troubleshoot an O2 Sensor

1. See if the check engine light is lit on the dash. The check engine light indicates that there is a problem with the computer controls of the vehicle. If the light is on, plug the diagnostic cable of a scan tool into the diagnostic connector of the vehicle. You will usually find this under the dash, close to the steering column. Follow the scan tool's instructions on how to read the codes that the engine computer has stored. If any of these codes describe an oxygen sensor fault, you have trouble in the oxygen sensor circuit.
2. Unplug an oxygen sensor from the wiring harness. Connect two of the leads from one channel of the oscilloscope or graphing meter to the oxygen sensor wiring. Start the engine and let it heat up; the sensor will not operate at its normal voltages until its temperature reaches 600 degrees Fahrenheit. The meter or scope shows the voltages measured over time, and graphs them.
3. Watch the graphing on the scope or graphing meter. You should see a wave pattern, moving up from about 0.2 volts to 0.9 volts, and back down to 0.2 volts with an even pattern, or waveform. If you see the pattern change radically and not in a uniform fashion, or if the voltage suddenly drops to 0 volts, it is a sign of a failed oxygen sensor. It is possible for an oxygen sensor to run far enough out of range to cause concerns, and still not trigger the check engine light.
4. Repeat the scope or graphing meter check for each sensor. If all the sensors test normal, inspect the wiring from the connector to the electronic control module for breaks or shorts. You can do the same test with the scope or graphing meter where the wires connect to the engine computer. If the sensor tested OK at its plug, but not at the engine computer, there is probably a wiring concern.