I must have gotten their one bum O2 sensor then. It's a Denso that's acting very lazy after 20k miles in my Buick. It'll stay high voltage for 10-15 seconds, then slowly swing down to low voltage for another 10-15 seconds. It'll then flip every second for 5-6 seconds, then go back to being lazy.
No CEL, since GM programmed the CEL to come on after 60 seconds of high voltage, low voltage, or slow switch speed. It'll switch often enough to not trigger the CEL.
A Delco sensor is on order. I'll be steamed if it's a reboxed Denso like the one I put in 20k miles ago.