So premature celebration ... my CEL came back on. It took longer, but did come on. I am getting a confirmed P2098 - Post catalyst Fuel Trim System too Lean, plus pending on P0153 - O2 sensor circuit slow and P0157 - O2 sensor low voltage. I replaced three sensors, but was unable to break free bank 2 / sensor 2 on the exhaust past the pre-cat. So, I can take another run at that this weekend & double check tightness and connections on the other three. Any other suggestions or things to check with this combo?
That same sensor was difficult for me to remove, also. Someone in this thread mentioned spraying it with penetrating fluid (something like PB Blaster or Liquid Wrench), let that sit overnight, then crank the engine and let it run for about ten minutes to warm the joint up. Heating the joint causes the metal to expand, loosening the threads and potentially breaking the seize. After you turn the engine off, get a breaker bar---not your ratchet---and try removing the sensor again.
After you install the new sensor, unplug the negative battery cable for the rest of the day (like eight hours or so), then plug it back in and go for a drive. The technique I used was ten minutes of city driving (up to 35 mph stop-and-go), ten minutes of highway driving (55--60 mph min/max; don't get out of that range), and ten minutes of city driving back home (up to 35 mph stop-and-go). After that, you should be good to go.