Check engine light on? Is car still driving normally? Go to auto zone and they will check the codes for you. Then post them back here so we can help you fix it.
I would probably guess that it is an O2 sensor... Those little guys can be quite annoying to change. But @dandyoun you are right. You need to go get the codes to really pin point it.
Search Lexus GS 300 Questions
Lexus GS 300 Experts
Related Models For Sale
Used Cars For Sale