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Leave the green wire connected but roll the rubber boot up so the contact is exposed. Connect one end of the voltmeter to the green wire contact and the other to a good ground on the frame, such as the strut bolt. The O2 sensor behaves in a binary fashion, toggling from ~.1V to ~.9V and back again as the ECU continually adjusts the mixture and hunts for the ideal setting (a 50% duty cycle).
Another test is to disconnect the green wire and measure the voltage supplied by the ECU. It should be close to 0.5V.
Note that the engine must be warm so that the engine is operating in closed-loop mode.
A) If you have a reading that is constantly low (<.5V) the engine is running lean. If you artificially enrich the mixture (propane, starter fluid, do at your own risk) you should see the voltage go above .5V.
B) If the reading is constantly high (>.5V), disconnect one or more vacuum lines to artificially lean the mixture. The voltage should drop below .5V.
Since the O2 sensor is fairly new it is most likely still okay, and is reporting a valid lean/rich condition to the ECU because of some other fault upstream. But if you can't get the O2 sensor to change state in A or B above, the O2 sensor is most likely bad.
Otherwise just continue testing normally for a rich or lean mixture, whatever was indicated by the testing above.
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