OK, time to jump in on this thread. I have an 2003 RX300 with 113,000 miles.
When the check engine light came on, I took it to my local guy who had recently replaced the timing belt and water pump. He checked the codes, but had nothing, so I went to another local garage I trust. Codes showed a Bank 1 lean condition. Since plugs were way past due, I had them put in new ones at a cost of about $300. The three back plugs all were very carboned up, and the mechanic told me that was the sign of a bad O2 sensor. I found a Denso OEM (234-9009) part on Amazon for $100, and with the help of YouTube had it installed in about 45 minutes. Reset the codes and off I went. About 20 minutes later the light reappeared.
Now I am really flummoxed. I've inspected the air box, and found nothing loose. Any suggestions as to what I should try next, besides taking it to the Lexus dealer of course.
Thanks!