Nissan Repair: P0420 Code Present, p0420 code, o2 sensors

Question
First, thanks again for the advice on the possible timing chain noise on my 1999 1.6L Sentra.  I've changed the oil to full synthetic 5W30 and now the noise only happens immediately after cold startup and no longer happens when starting a hot engine.  It used to happen after starting hot or cold.  So maybe the thinner and very slippery oil is starting to free  the tensioners.  I hope so.

   New issue.  The check engine comes on setting a P0420 code (catalyst effeciency below threshold, bank 1).  If I turn the light off, it comes back on in 2 or 3 drive cycles, so the problem is persistent.  I know that P0420 is set when the ECM sees the rear O2 sensor tracking too closely with the front O2 sensor.  So after driving for an hour and getting everyting at normal operating temperature I put it on a scanner and ran the engine at about 2100 rpm for a minute or two to watch the O2 sensors.  I did the test four times.  Between tests I let the engine idle for a minute.  

   All 4 times the front O2 sensor switched about every second or two from about 200mV LEAN to about 800mV RICH.  That seems normal.  The rear O2 sensor behavior was puzzling though.  The first two times the rear O2 sensor never switched but rather stayed very constant at about 400mV LEAN and very slowly rose to about 500mV LEAN.  The O2 heater was always OFF.  The next two times the rear O2 sensor again never switched but was now at 700mV RICH and rose very slowly to about 800mV RICH.  The O2 heater was now ON.  I'm not sure how to interpret these results.  Is there a problem with the rear O2 sensor?  No O2 sensor codes are present, only P0420.  Also, why is the light coming on if the two O2 sensors are operating so differently?  I'd appreciate your thoughts as to what's going on and what the problem is.  Many thanks again.  

Answer
Al,

The tensioner should continue to get better.  If it were me I would change the oil every 1500 miles until it went away.  Remember that you are really cleaning the passages with the new oil.  On the oxygen sensor I would think a good fuel injector and new plugs might get rid of the converter efficiency.  The readings being different are normal since one is in front of the converter and one is behind.  You probably have a couple of things going on.  The first is the injectors are probably gummed up and you might want to put plugs in it to burn the mixture better.  If these fail you are probably looking at a new converter.