Easy 02 test for DIYer is to hook the meter to sens sig wire and ground ... unplug sens sig wire feeding back to ECU ... leave sens heater wires connected ... warm car to normal...
Now , pull vac hose , creating a vac leak.. the sens should go lean
[ low v].... correct vac leak ... cuff the air intake, choking the engine and creating a rich mixture ... the sensor will see rich and read high.
When doing either of these , the meter should move as you open/close both vac leak or choke intake air..
All is you are doing is using the engine as a mixture tool and seeing if the sensor responds to your manually changing the a/f mix...that is why you do not want the wire feeding back to the ECU , as the ECU will try and correct your manual a/f mix inputs .........
you are looking for .1 to .9 volts, lean/rich , with a 1/2 sec change time...
that would indicate a decent sensor