I replaced the "after converter" sensor when the check engine light came on. What a bi*ch trying to unscrew it.
Then I replaced the converter a week later. And wanted to replace the "before converter" sensor. I went to NAPA and he said the same thing about passenger or driver side. One was over 70.00 bucks and the other was over50.00 bucks. I bought the higher one, and noticed that the plug had the notch off slightly, but the wire length was the same. So I go back to NAPA, return it for the shorter wire that HAS the notch in the same spot as the old one. When I showed it to the muffler guy putting the new converter on, he said that will never reach. So, I just ended up putting the old sensor back in the "before the converter" spot.
Anybody replacing the upstream sensor, check the notch location with the old one, and see for yourself !