I had this same problem. We bought our 2000 RX used and shortly after SEL came on I think same lean code. After several resets at repair shop O2 sensor went bad and replaced it. Soon after SEL came back on but it ran OK so I didn't get around to having it looked at. Several months passed and then things started acting up, sommetimes loss of power,transmission hunting for a gear on acceleration and not always shifting to OD. Shutting off engine seemed to fix it for awhile. Took it to Lexus , they replaced the MAF sensor which fixed the loss of power and gear searching but not the OD problem. They talked about the fluid temp sensor but it checked out OK. I had it back to them three times and finally had a mechanic go with me for a drive where he witnessed the problem. They tried clearing the memory,consulted lexus tech assistance and finally at techs suggestion replaced the tranny. The OD problem was still there!!! Finally after 2 weeks they found the knock sensor was faulty and replaced it. PROBLEM FIXED!!!! I don't know how much the sensor costs, they were so embarrassed they warrantied the sensor,tranny labor and threw in some other stuff as well.
