There's 4 sensors. 2 before and 2 after the cats. Generally, you warm up the truck, put a meter on a sensor, at idle, and take a reading. S/b something like .5vdc. Run the revs up to about 2500rpm. You should see the voltage run up to 1vdc, or so. This is a good sensor. A bad sensor will read low or zero and won't react to the rpm change.