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P1155 is bank 2 sensor 1 air fuel ratio sensor malfuction. That is the sensor in the exhaust manifold closest to the radiator. When replacing make sure you use a foctory Denso sensor. The part number from Lexus is 89467-48011, and will cost around $200.
Make sure to spray the threads with some penetrating oil and let it sit for a while. Then before loosening tighten it just a bit to help break the rust free.
I hope my answers are helpful for you with your check engine light. That is a very common problem on that model. If you have any more questions you can post them here or request me at any time.