Greetings Kay,
There are 2 A/F ratio sensors. one on the rear bank (B1) and one of the exact same sensor on the front bank (B2). I wish you had posted what the CEL fault code was. May I say one thing, a lot of companies make A/F sensors for the RX, but I wouldn't use ANYTHING but the original Denso sensor. They can be purchased on Amazon for about $100. Some of the other brands don't play well with T/L and MAY come back with a CEL when it's the sensor not interacting because they don't know how to make sensors as well as Denso does. This site is a very good site for OBD2 codes and the link is LEXUS specific. http://www.obd-codes.com/trouble_codes/lexus/
Be forewarned that there is a mistake on the site though. P1155 SHOULD say (B2S1 instead of B1S1) BOTH front and rear sensors are the same sensors, both located shortly after it comes out of the exhaust manifold. The front one is VERY easy to see and the rear has to be accessed from the bottom, back by the firewall. If you had a code for an A/F ratio sensor, that code should be SPECIFIC to THAT sensor. If you are getting the SAME code, I strongly suspicion it may be the aftermarket brand sensor. Please post what the original code was.
Edit: Kay, the info Joe gave you is incorrect, the front one is Bank 2, sensor 1, NOT Bank 2, sensor 2. Do a search here AND on Club Lexus, there is quite a bit on changing the sensors. Unfortunately, the rear is much more difficult to change because of where you have to release the plug on the pig tail. There ARE good instructions on how to do it without cursing a blue steak though. DO read up BEFORE you try to change it though. There are also good pictures to show you exactly what you're looking for.
A male named Kay? Please tell me it isn't so! Is that a little like a boy named Sue? :)
Thank you code58 for correcting my error.