Perhaps when the O2 sensor was changed the wiring was damaged. A very good mechanic that I used to know always used to say "start from the last thing that you changed" 90% of the time he was correct.
The first thing I would check is the wiring leading to the O2 sensors , particularly where it crosses any body work. If nothing is obvious I would try simply disconnecting from the sensors, in case there is some weird problem with one of the sensors.

Peter

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