It is best if that tool can graph the O2 sensor behavior. Supply that if available.

The older style of O2 sensors that we have sit in front of the cat and determine if the air fuel ratio is lean or rich and then tells the PCM to do the opposite. The mixture then bounces back and forth around the ideal 14:1 ratio, the average of which is correct. So its a closed feedback loop that is always switching around the target level. The front sensors are always changing voltage into their upper and lower limits of 0 to 1.0 volts or more precisely 0.2 - 0.8 ish volts. Graphic is better because the speed of the square wave and the limits are what you need to evaluate. The rear sensor monitors what the cat output looks like as a check on cat efficiency. It should be more stable around the ideal level of 0.45 volts which equals the ideal air fuel ratio.

Another thing that is done is to introduce a lean or rich condition and see if the sensors and system respond correctly.

George