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The rear O2 sensor is there only to monitor the catalyst function. The front O2 sensor sweeps between about .1-.9 because the O2 levels fluctuate as the mixture control circuit goes from lean to rich. With that being said, if the cat is doing it's job it will consume all of the left over O2 in the process and the O2 content that comes out of it will be at or close to a fixed level. If you artificially enrichen or lean the mixture beyond the control unit's ability to compensate then you should see a change in the output voltage of the rear sensor but it should pretty much remain constant at whatever voltage it is at instead of fluctuating. Typically though you should see a number closer to .4-500 mV.

One of the accepted ways of checking O2 sensor response is to bring the idle up, feed it just enough propane to peg the O2 sensor rich, shut off the propane and rap the throttle once, it should be able to drive the mixture from rich to lean in less that .300 mS. Obviously to do this test you will need an oscillioscope and know how to use it.

Mark