Toyota Repair: PO 420 2000 toyota tacoma California Emmission, cat converter, proscan

Question
Hi I have had this code for just short of a year at first my proscan show it as catalytic event below threshold bank1
i was informed that this could be the converter so i started by replaceing the first converter ( this truck has two) and cleared the code it came back about 100 miles later i then replaced the o2 sensor before the catalytic toyota calls it a fuel ratio sensor again cleared the code and again it came back the only factor after this was my fuel economy greatly
improved at this point i ran another proscan and the PO420 however it know read Catalytic efficenty low so i fiquired
it might be the other converter as well as a notice white power in the tailpipe which i was informed could be residue of a failed converter . i replaced and cleaerd the code again and it came back as the same code .the vehicle runs great as well as the rear o2sensor voltages appear to be what they are supposed to be seeing i have replaced most of the catalytist system i am at a total loss and am seeking any help in resolveing this problem i have also been told if a code is in the cars computer to long that it has to be reprogramed ??? I would appreciate any help in this matter after 30 years of repairing my own cars i think i the car has won also i was told useing a pyrometer on the cat shows if they are lighting up the front one was and the rear cat not prior to being replaced

Thank You
James Brown

Answer
The computer is ROM only and is not reprogrammable to erase codes. Since both converters have already been replaced(with OEM parts) I hope and the rear Os sensor has never been replaced it may be worth a try to replace it, without actually seeing the voltage waveform of the rear sensor myself I can't judge whether it it may be considered good or not, the rear sensor should stay around -1volt when in closed loop, the sensor may be reacting too slow to the changes of the cat converter reaction and so the computer will trigger the check engine light as a cat efficiency below threshold.