do a search on "oxygen sensors" and you will find many threads. Bank 1 Sensor 1 is the harders to get at. It is between the cylinder head and the cat. on the rear bank of cylinders. It is a tight squeeze to get your hand in between the exhaust manifold and the firewall.
The sensor will cost over $200 from the dealer and then another 1 to 1 1/2 hours in labor (at close to $100/hr.) if you have it done at Lexus. A toyota dealership may be cheaper as the same sensors are used in many Toyota models.
The sensors can be had online for cheaper and you could do it yourself if you have the tools, time and knowledge. If not, seek out a reputable independent mechanic in your area.
When the trouble code (P1135) was read, they may have cleared it, thus the CEL being off. It may come back. The sensor may not be totally faulty, just on its last legs.
The last point I must make, is that the trouble code you get may not always point to the culprit at hand. Problems further upstream from the indicated trouble spot may trigger codes from different sensors. It takes a knowledgable diagnostician to figure it out. Replacing every part that every code points to can get expensive.
Good luck.
steviej