03 Impala Cat question

'03 Impala, 3.8 66k miles, P0420 code Started out as a plugged cat issue, cat was replaced by his mech. .(brother's car and I don't wrestle with exhaust work on jack stands)
Low cat efficiency bank 1 code popped up a few days later. Changed front O2 sensor, fine for a few days and same code came back. Changed rear sensor thinking it may be off enough to trigger a false low efficiency code but not a fail code and possibly got damaged when it was removed during the cat change out. Fine for almost a week and now the SES is back on again. Haven't had a chance to read the car yet but I'm betting on the P0420 being back again. No other codes were set with the P0420. Pro's wadaya think? New cat bad?
TIA John......
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