FWIW...Testing o2 sensor

With a 4-wire oxygen sensor, the output signal works in a constant
loop, essentially, the fuel injection computer sends a signal to the sensor,
and the sensor then sends the signal back to the computer. This is achieved by
having one wire carry the signal to the sensor, and then another wire carries
the signal back to the fuel injection computer. You also have a wire carrying
12 volts to the heating element, and the last wire is the ground for the
heating element. With a 4-wire sensor, you will
need to measure the voltage fluctuations between the two signal wires. To do
this, start the car and let it warm up. Next, disconnect the oxygen sensor and
measure the voltage between the wires. Now take off the oil filler cap to
simulate a rich running condition. Watch the voltage, if it begins to fluctuate
rapidly, then the sensor is good and your done. If it stays the same,
o2 sensor probably kaput.

Low voltage means there is oxygen in the exhaust (oxygen on both sides of the
sensor) and the engine is lean. High voltage means there is no oxygen in the
exhaust and the engine is rich. An old worn sensor has a low output which
causes the computer to enrich the fuel.

thats great. but most these run 3 wire.

3 wire sensor is probed the same. Find the signal wire. Probe for voltage. You can very easily probe the signal with a sharp Multi Meter probe without disconnecting if you desire. Harbor Freight cheapish at $3.00 or free with coupon, grind the tip to sharp point works for me. Throw one in DD trunk tools every car. Saved pain and anguish with 25 y.o. machines.