Start the engine and get it at operating temperature Closed loop. Back probe the signal wire which is the braided wire to the positive side of a voltmeter. The negative side of the voltmeter should be attached to a good ground; the battery. Allow the engine to reach operating temp. Closed loop. Rev the engine to 1500 rpms. A good 02 would have a voltage reading of 200ma lean to 900ma rich. Any rapid reading in between 200ma and 900ma means the 02 is good. If the reaction of the sensor is slow, or the voltage never reaches rich or lean, it means the sensor is bad.