On a 92 SC400 (1UZFE in general) there are 4: 2 upstream and 2 downstream. the Downstream has a very long lead while the upstream has a short one. The downstream (or whatever is on the exhaust pipes, easily seen from bottom) takes an open end 7/8 wrench and a good kick with your foot on the wrench to loosen, an 02 socket WILL NOT WORK on these. The harness goes up under the carpet via a rubber grommet and into the cabin. pull carpet back and unplug. When you reconnect the harness, plug in and give the new O2 3.5 turns CCW before you start threading it in CW so the harness will unwind while you are tightening it, smart eh? (From service manual...) or just install the O2 first and then pull harness into cab and plug in. The other one is located on the exhaust manifold above the catalytic convertor and is barely accessible from under the car, ideally youll need a 7/8 crows foot and a breaker bar with a 12" extension but whatever it takes will work. use penetrating oil first and remember to break these so you wont run your hand into anything once it lets loose, good general practice! I got all 4 for <$90 on Amazon.
