Testing resistance is not really a good test when a sensor has been installed and used. You are much better off testing voltage when the car is running. O2 sensors send voltage back to the ECU >0.5v equates to a rich mix and <0.5v will mean a lean mix. To provide these figures, the sensor will sample oxgen from the inside of the exhaust and also from the outside, so it is a good idea to make sure that the outside of the sensor is clean. Also when you test, make sure that the car is up to temperature, so get your probes into the connector prior to starting the car. It would be handy for fuel trim figures to be read. Also a fuel pressure balance test will let you know if you have a leaking injector etc. Hope this helps.