So on my 2004 SE I replaced the cats and the Y pipe and both A/F sensors and both O2 sensors. I've driven it 100 miles or so since and today the CEL, my dreaded friend, is back. The bank 2 cat I installed is a Walker and the element is a smaller diameter than the original. Why did it take so many miles for these codes to trigger? Is there a reasonably priced scan tool that would give me more details on the cause? Could a standoff for the O2 sensor possibly turn off the CEL with no ill effects from driving in this state? The scanner states Heated O2 sensor Bank 2 sensor 2 for both P1166 and P1167. Is this probably accurate? Are all the codes pointing to the same sensor? Could the proper scan tool tweak the tolerance?

Yep, ESM says those are all voltage codes for the Bank2 rear HO2S. A spacer may help, but with those codes I think it's more likely a defective sensor or a damaged harness.

That sensor has about an 18" cable and the length needed is about 6" so it is coiled up and stuffed in. Must be a reason.

Lots of aftermarket HO2S's claim to fit multiple applications but in practice fit badly and don't work right. I went through a bit of that with my old hoopty Hyundai. I finally bit the bullet and bought an OE sensor after going through two crappy eBay sensors and a supposedly-decent one from RockAuto, none of which worked right.

My Hyundai Dealer sold me Bosch part stuffed THEIR "Hyundai" box , was the only part that worked - didn't return codes , whereas A-zone , NAPA , and Rockauto's ALL couldn't cut it !!
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I'm experimenting too probably wasting money but I couldn't resist. I'll report back.
https://www.ebay.com/itm/144286803873

I'm not able to get into forum with Firefox. There are posts missing when I use Microsoft Edge but at least I can get in. So I spliced in the new socket and cleared code and for a day of driving no CEL returned. I need to get a scanner that gives real time data on how efficiently systems are operating.

Dunno what's up with the site, but it's been on-and-offline since Thanksgiving and all the posts since 11-19 seem to be missing. If they're struggling with an incomplete "restore from backup" then those posts may be gone for good.