Finally got around to replacing the rear O2 sensor this weekend b/c inspection is due. I have to admit it took me a while to find the sensor. I am use to the downstream O2 being farther down the pipe – typically mid car. I had to Google the sensor to find the location and was surprised to see it is so close to the primary.
Working under the car on ramps, I sprayed it with Kroil and let sit for ~10 mins. Piping was still very hot and I can hear it fizzle/dissolving upon contact so not sure how much good it did; didn’t have time to wait for it to cool but I should as it was still hot on my arms. I had an O2 sensor socket (22MM, can borrow from Autozone) to break loose sensor with a ratchet + extension pipe. You can reach up to unplug the sensor with both hands. Everything came off easily. There appears to be a nice layer of anti-seize on the OE plug. The part that I did not want to waste time on and can be a nuisance is the bracket that suspensions the wiring from hot exhaust parts. The bracket is mounted to the exhaust manifold by a 10MM nut (rusted but can take off) and on top of the bracket is clamp that secures the O2 wire with a 10MM nut on a carriage bolt that has a round head -doh! This nut is also rusted + a soft metal bracket that deforms easily means you can’t easily take this off to easily to reuse. If you want to do it proper, plan some time to dermel off the nut and replace with a new nut and bolt OR get a new bracket in advance. I choose the easy way and just snip off the old wiring at the bracket, wrapped the new longer than necessary wiring around the bracket, zip tie, and be done.
I did not disconnect the batter so it seems upon removing the old sensor and install the new, it triggered P0139 (pretty much the same thing as P0138). Rest light and no CEL for tad over 200 miles now (it would have come back by now). Nice to not have to see that red light.
I can safely assume the old sensor was done at ~30K. The wiring looks good, no loose connection, nothing visibly burnt, and the connector plug is clean as a whistle.
Update: passed inspection and now over 300 miles and not light. yay!