Toyota Repair: 2000 toyota tacoma with california po420, o2 sensors, apprecated

Question
QUESTION: I have had a po420 code which i thought i had solved i have replaced the upstream and downstream o2 sensors as well as both catalytic converters the thing that finally solved or so i thought was the replacement of the rear o2 sensor after the cat i cleared the code and was good for a few months and several thousand miles now it has returned i reset the code and it returned in 80 miles i am due for emmision this month and will fail i basically do mostly city driveing and have read this is not good for the emmision system i would appreciate and advice on what could solve this problem the engine runs well and has plenty of power i have been working on cars for 30 years and have run out of ideas
Thank You

ANSWER: Did you replace the cat right behind the exhaust manifold on the right side? This is a V-6 is this correct?

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QUESTION: this is a 2.7 4 cyclinder and i have replaced basicly the entire catalist system the truck runs well and i was wondering  what could damage the o2 sensor in such short time there was a local issue of bad gasoline in my area could that have possible caused it i ran a proscan on the rear o2 sensor and the data looks like it could be damaged it is appx 2 months old it was factory toyota part
any ideas were to look would be apprecated
jim

Answer
Contaminated fuek can very well ruin an O2 sensor, what did the waveform patterns of both of the sensors look like, the front sensor should change voltage at about 8 times in ten seconds, if the rear sensor is working correctly and the cat is working it's voltage should remain relatively flat at about .1-.3 volts, if the rear sensor voltage follows the front in step then the cat is not working, keep in mind that depending on what contaminated the fuel the cat can also be damaged idf driven for any lenght of time.