Just thought I'd pass this along to the DIY'ers here. I stumbled on it while trying to figure out whether I have a bad cat in my 2000 Tundra V-8 ($2K replacement cost). It threw a P0420 code (Bank 1 catalyst below efficiency threshold) a week or so ago. After clearing the code, it has not returned again yet. I did notice that my O2 sensors are showing some suspicious voltage readings and graphs with my scanner (ACTRON CP-9580). Which led me to this article:

http://www.picoauto.com/applications/lambda-sensor.html

Hopefully, it will give some of your ideas for troubleshooting a problem that has you stumped.

 

Here's another good reference from my DSM days. The diagram in the link shoes you how to bypass your rear O2 sensor. I'm not even sure if the wiring is similar between mitsu and honda, so use at your own risk

http://www.vfaq.com/mods/O2bypass.html

 

Interesting info on the heated O2 sensors. I have a code on my Sienna (P0135) that is a "o2 sensor heater circuit". I just haven't gotten around to checking into it further since it's just the heater circuit. I need to see if the heater circuit is separate from the sensor or not. Based on your link, I'm guessing not.

 

The heater in the sensor usually fails open.