Thanks for the reply -
It seems, from what I can find, that the P0125 code is being thrown because of a bad A/F sensor. Since I've gotten 1155 & 1150 (pending) I have no idea which sensor is really the troublemaker. I've got no way to test the sensors, so it looks like I'll have to pay to have them checked...bummer. Good news - there's a Lexus shop that's cheaper than a dealer. Bad news - it's about 50 miles away.
I have to say (venting here) that the RX300 has begun to have too many gremlins IMO. Knock sensors went bad...replaced & then the OEM replacements were bad. Three of the coil packs have failed. I disconnected the sensor to the brake fluid reservoir because it failed and shows low fluid. The servo for the A/C mode has gone nuts. Lots of failed bulbs, etc. In contrast, my '93 Acura Legend is running strong without any failures of any kind. No CEL, all the original stuff works...I'm just sayin"! I do love driving the Lexus, though.