You didn't mention what year, or how the car runs, mileage on car, mileage on the o2 sensors, actual voltage swing from sensor 1 when sensor 2 is very low, if there are any exhaust leaks before or just after the sensor, etc. It is impossible to even guess. Usually a P0420 is a bad converter but the readings you are giving are not the normal readings for a converter failure. What are you using to get these readings?

Paul

Edit: I see that torque app is an android app. I don't know anything about it but I guess it works. I have seen some of the cheap (less than $200-300) scan tools that weren't very good when working on problems like this. The one we were using before we retired was somewhere around $8000. (Snap-On)

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84 500 SEL (307,xxx miles)