Lexus Repair: oxygen sensors, lexus rx 300, oxygen sensors

Question
My Lexus  rx 300 the check engine light is on . I took it an they said it is the oxygen sensors . I had 2 of them replaced ,I think their is four.Do you know where they are ?  

Answer
Two primary sensors, one on each manifold.
One post-cat o2 sensor that indicates to the ECU when the cat converter has kicked the bucket & the car is letting out high emessions.

Scan the computer & write down the code. It tells you which one is outside of it's specifications.