I had a check engine light come on and when I diagnosed it was code 25 which is upper oxygen sensor (s)....I had a spare engine that was bad so I pulled the oxygen sensors off of it to replace the ones on my latest lexus..92 SC400. The check engine light would come on usually right after starting the lexus up....or once you revved it up and it would stay on....
I made the repair to the drivers side first as I was also checking the oil level sending unit, the oxygen sensor is just to the right of the oil sensor....
So to change out the oxygen sensor,
1. Jack up the driver side front of car, place a jackstand under it for safety and then remove your tire and wheel.
2. Looking straight inside the wheel well opening you will see an opening into the lower engine area, straight ahead you see the oil level sending unit, and to the right the oxygen sensor...mounted to manifold area of exhaust.
3. Disconnect the connector for the oxygen sensor and then using a 22mm wrench remove the sensor. It is in a tight spot so take your time....I even had to slip the box end of the wrench up the wiring of the sensor to get a better grip on it and then remove it. Once you get it started then it will come right out.
4. Once you have it removed then go ahead and clean the connector end that is still attached to the engine by spraying some electrical contact cleaner inside it and also the outside of it to remove any oil or other contaminants.
5. Now just reinstall in the reverse of removal.

I got lucky as this was the one giving the bad reading....so didn't have to do the passenger side. But the passenger side is in same place so just jack up passenger side and look in for the sensor...Took it for a test drive and no more Check Engine Light....

Good Luck, hope this helps