This was posted *long* ago, I realize, but did you ever solve your problem. My husband is having a similiar problem with our ES300. Thanks. :)
I've looked and looked can't find anything. I even ran across some information that indicates there are only two in the SC 300/92. Good luck and keep us posted.
My husband ended up cleaning the sensors and all the sensors now work fine. Our problem has been narrowed down to the wiring harness between the a/f sensor and the brain. Is the sensor for sure bad or just dirty?