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Hello and thank you for using just answer.
For some reason there is a common misunderstanding that 02 sensors cause false P0420 or P0430 codes, on Toyota vehicles this is NOT the case when the catalyst code is the only code sotred. For example if you had P0300 misfire codes AND a P0420 then the misfires could be causing the catalyst to overheat and malfunction, when you have only a catalyst code and no exhaust leks present this means that everything except the catalyst is operating normally (you need a new catalytic converter!).
Yes you are correct that with california emissions there are 2 inline on the pipe, sorry but you need to replace BOTH of these. THe computer uses the air fuel ratio sensor at the front and the o2 sensor at the rear of these converters to calculate total converter efficiency. Also be advised that you get exactly what you pay for, please do not make the same mistake that countless customers of mine have made and install aftermarket (cheaper) converters as the content inside is much less (thus the reason for cost difference) and especially on a california vehicle the P0420 code will continue to reappear, you need to get genuine Toyota brand parts, there are a few dealers that sell cheaper wholesale online such as www.metrotoyotapartsnow.com if you are trying to reduce cost here. Also if possible you can return that sensor for your $$ back for an unneeded sensor.