Over the years of reading about how the rear O2 sensors work I've become nore confused or thought what I read was wrong. Now I think there might be 2 differant stratigies and hope someone can clear it up?

O2 sensor 1, bank 1 and 2. Both work the same way as OBDI with switch points and adjust fuel for closed loop.

O2 sensor 2, bank 1 and 2
1. Newer OBDII vehicles seem to use the second O2 sensor to check CAT efficiency with no CL fueling feedback?
2. Older OBDII vehicles second O2 sensor behind the CAT have some limited fueling correction? I can confirm this reading a 98 Vortec dissasembly as there is highs and lows for CL feedback on rear O2 sensors, also a bit for duel exhaust or single exhaust. Dual exhaust has dual CAT, Single exhaust has single CAT.

Reason I even got involved was my 98 Vortec has some fueling issues at idle. O2 sensor readings are:
Bank 1 sensor 1 switching and looks normal
Bank 2 sensor 1 switching and looks normal
Bank 1 sensor 2 steady voltage from 500 to 700 mv
Bank 2 sensor 2 voltage at idle swings like a sensor 1. When engine is reved it will go steady voltage. What alerted me to the issue was STFT and LTFT on bank 2 way off at idle only! It may be a old lazy O2 sensor or I have one bad CAT? Would a CAT go bad just at idle? I've done the on board O2 sensor tests and all pass!

At this point I think I could get better fueling from single O2 sensors? But will have to test disconnecting them and turning off diagnostic tests will allow front Pre CAT O2 sensors to work properly. Turning off diagnostic tests (not just MIL light) only and I still get readings from the post CAT o2 sensors.