Dual O2 Sensors and how they work?

[EagleMark]

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.

[1project2many]

One bad cat for sure. Partly plugged or partly blown open can affect sensor temp and response. Did you have a misfire on bank 2 that might have caused partial meltdown?

Later vehicles will alter fueling from post cat O2. It's an oxygen in the catalyst thing.
Early do not or do less.
Unsure where early ends and later begins, though.

[EagleMark]

No misfires since I have owned it.

Injector spider was replaced along with intake gaskets, cap rotor plugs and a ton of other work about 6 months before I bought it. Total of recent receipts was $4000. and it also came with service records since new!

I just remembered when I got it I replaced 3 O2 sensors because I could not find records of them being changed and 3 is what the book showed.... taking a deeper look today there is one more post CAT O2 I did not change. This may be the issue?

Anyway to check CAT on vehicle?

[ony]

switch the o2s in the after cat position if the wires are long enough see if that bank has the same reading, if it has then the cat might have trouble. if it is a seperate duel ex. sys it may have one cylender running rich or lean.

[JeepsAndGuns]

Injector spider was replaced along with intake gaskets, cap rotor plugs and a ton of other work about 6 months before I bought it. Total of recent receipts was $4000. and it also came with service records since new!

Makes me wonder what was wrong with it to require all this work being done? Possiably the injector unit was bad or sticking on the side in question and therefore made the cat start going bad?

[EagleMark]

Makes me wonder what was wrong with it to require all this work being done? Possiably the injector unit was bad or sticking on the side in question and therefore made the cat start going bad?

Not really hard to get to that amount with GM dealer service, all service on truck was what GM recommened at milage for life of truck without regard to if it needed it. Main issue was Intake gaskets were changed, fairly normal with DeathCool anti freeze, full flush and green stuff. Spider injector while it was out, oil cooler lines, transmission cooler lines, AC high pressure line, spark plugs, cap rotor... new tires and some other stuff. This did not include the new transmission 1000 miles before that. Truck now has 116k.

As a follow up I installed the other post CAT O2 sensor I missed the first time and the issue is gone.

[1project2many]

I just remembered when I got it I replaced 3 O2 sensors because I could not find records of them being changed and 3 is what the book showed.... taking a deeper look today there is one more post CAT O2 I did not change. This may be the issue?

Failure to provide all information can lead to incorrect diagnosis. ;) Although I don't usually run into an issue with a lazy heated, post cat O2 without a matching code on the vans. Are the O2 heater codes enabled in that cal?