OBD II code P0420

I read the two threads that were available on this forum, but I am not sure they apply to me.

Believe it or not, in our 98 Transport (that ran great), we just finished a 25,000KM trip that started in Vancouver, BC, took us to Newfoundland, Key West and back home. All in 10 weeks! Great trip, but won't be doing is again anytime soon. (Did 1 mention 4 young kids)

The Engine light came on in Miami after an oil change at Firestone (I also ran out of gas on the Florida Turn-pike the day before). I read on the forum that I could reset the code by disconnecting power and that worked for about 50 km then came back on. (An american friend had a reader to get the error code but coould not get it to work.) Got the code back at home. P0420

Leaded fuel was used where unleaded was called for - Not possible
An oxygen sensor is not reading (functioning) properly - need to test
The engine coolant temperature sensor is not working properly - It is fine
Damaged exhaust manifold / catalytic converter / exhaust pipe - No damage
Retarded spark timing - ECM controlled

Vehicle just passed Provincial Emmissions test with reading slightly above normal.

Should I ignore the error code, or are there more details available if I get access to a proper code reader.

could be the cat converter going south, but i doubt. try changing the O2 snsor first, it's a lot cheaper and more likely the problem.

Thanks for the input.
I diconnected the sensor after the Cat and tried to measure the mV. Looking at some information on the web they said the reading should be 200 mV. Any variance indicates it needs replacement. Is this correct? I tried to get a reading, but there are 4 connections. Did the readings from all possible combinations (with engine running), but only got a couple of mV at the highest in one combination.
Am I going about this properly?

Thanks for the input.
I diconnected the sensor after the Cat and tried to measure the mV. Looking at some information on the web they said the reading should be 200 mV. Any variance indicates it needs replacement. Is this correct? I tried to get a reading, but there are 4 connections. Did the readings from all possible combinations (with engine running), but only got a couple of mV at the highest in one combination.
Am I going about this properly?
The code is set by the pcm comparing pre cat to post cat readings the post cat reading is supposed to switch at about 1/2 the frequency of the pre cat sensor, this requires a graphing scanner to really diagnose. The post cat sensor usually holds fairly steady at 450-600 mv if the engine is running right and the cat is working. Check this link first, http://www.troublecodes.net/articles/catfailure/