Stage 1: occasionally the sensor gets stuck at a particular value, in this stage the PCM generally trusts the oxygen sensor absolutely and will set P0420 and Evaporative emission codes, even, and if it's analysis from when it's briefly stuck this could really send you down the wrong lane of diagnostics! Hey, maybe that's why snake oil "fixes" some P0420's or catalytic converter failures? (P0420/P0430 for bank 2)
O2 sensors the 4 Stages of Death*:
Stage 2: the sensor gets stuck more often or, more often needs more drastic changes from fuel trims to respond, but the PCM finds the sensor still useful. Yet, it trusts it less and will often "trill" the sensor like a Soprano Opera movement up and down in micro changes, not enough for stoichiometry. Often in a relearn, I see this, but it can be the PCM distrusting. New sensors can sometimes simulate this stage of death, but really the PCM is just testing what it's working with (not truly like an opera singer unless she was continuing up the scale).
Stage 3: this is when the sensor goes out of range. It's normal on cars, in my opinion, to be able to read below 100 millivolts when they're not yet up to temperature, don't be fooled that this is a stage of death.
Advanced degrees of stage 3 may include 1.2 volts, flatlining at zero volts, STFT maxing and and LTFT maxing out to get a result? Then stage 4 isn't far behind.
I once described some of the above less refined with Scanner Danner, he responded that an unplugged sensor will behave the same. The car will go in limp mode. It won't try to change fuel trims anymore. It's been beaten down to a carburetor without different levels of jets to change and meter fuel use, the engine limps in shame.
Note: this is for the sensor side of the oxygen sensor only. There is also a heater circuit that can cause issues. I don't know whether this circuit has played into the stages of death or not, but no codes have come up for heater failure on my ride since nearly 140k miles ago when I was a parts changer and changed my upstream O2 sensor on an engine code only.
Maybe 17-20 miles per gallon on a 27 mile per gallon ride for stage 4 of death. If I wasn't sick I'd have filled up my gas tank to see my gas mileage, early on the last Bosch showed marked improvements on gas mileage, but it sadly went wacko on me.
*The stages of death is of my own creation. The mere act of publishing this in America constitutes a copyright. This is the level of excitement I have for this subject of oxygen sensors. I made similar comments on my grounding wire upgrade.
I gave permission there to share this idea so long as you link to this video.
The making of this video would not be possible without Schrodingers Box.
Observe the rules of the road, in some states filming while driving is illegal. I keep looking at the road when I follow sensor data. (It's like when I look in my rear view mirror, when I'm smarter I'm STILL looking in front of me as well.)
Related link for O2 Sensor Downpipe Mod and Diagnosis - the easier way to change an oxygen sensor on our Chevy Prizms! (1998-2002 Toyota Corolla, except for some body and interior, it's the same car)
A recent article to help you more fully understand upstream O2 function
(Wow, that was my fifth article about O2 sensors, so there's even more you can find from within the article above.)
See a practical example of the 4 stages of death for oxygen sensors. I'm on YouTube now!
by AutoBravado, video is on the DE Nichols channel, also by AutoBravado