Not sure what an AF sensor is, but on my car, I have two O2 sensors in each exhaust pipe - one before the catalytic converter and one after. Although they are not the easiest things to reach, mine came out with normal wrench pressure (and that is with 195,000 mi on the car). Found it helped to warm up the exhaust system a bit (not so hot that you burn your hands, but hot enough to loosen the threads on the sensor).
Not sure if Bosch makes sensors for Lexus, but mine were $65 at the local auto parts store. Once you get at them, they are an easy fix. Also, Autozone (the car parts place) can read your computer for you and reset the error code for free. The sensors closest to the engine tend to die, while the ones further down the exhaust pipe tend to be OK.
Good luck.