O2 Sensor issues PO420 now PO136 - 2004 Tundra DC SR5 4x4

I've been fighting a CEL light for O2 sensor issues in my 04 Tundra Double Cab SR5 4wd. I bought the truck used with just under 200K miles on it. The previous owner didn't like to turn a wrench which worked out in my favor as all service was meticulously done and recorded by his preferred shop. He said the CEL light would flip on and off due to exhaust temps running hot.

After a couple weeks I saw the CEL light, which came back with code PO420 "O2 sensor bank 1.. Catalyst system efficiency below threshold bank 1". I proceeded to change out fuel filter and my air filter. Had the light turned off, it would turn on and off over several days intermittently. Then decided to replace both upstream O2 sensors (Bosch 13441). The CEL light turned off. Then back on with PO136 "Bank 1 sensor 2" for the downstream sensor. I replaced the downstream sensor. The CEL light turned off. Then back on with the original PO420 code. I had a shop replace both catalytic converters (Magnaflow 24404 and 24406). The CEL light remained off for a few days then flipped back on with the PO136 code.

I've run out of ideas on what to do to get this CEL light off. The only other thing I noticed was when the PO420 code was coming on I could fuel up on premium and get it to turn off. The idea of bad gas crossed my mind but the gas stations where I live haven't been known for bad gas and it hasn't flipped on any warning lights for my wife's or coworkers cars.

Any insight would be appreciated!

 

I have a 2004 DC with 210K miles that I bought in early December. It was throwing a P0420 code. I changed the up & down stream Bank 1 O2 sensors but it was still throwing the code after a few miles of driving. I used Denso brand sensors. I wound up putting all new cats on both sides (Bank1 & Bank2) and replaced the resonator, muffler & tailpipe since the exhaust system was all rusted out. I used Walker Ultra cats & exhaust parts. I have about 1K miles and haven't had any codes since. Cost me more than I wanted but at least the problem seems to be resolved.