Hello,
Thanks for using Just Answer.
Your LS does not have a sensor that monitors the actual pressure. The system uses the wheel speed sensor and the ABS system to monitor tire pressure loss. There are a few things that can cause the light to come back on. First could be improper initialization of the system. You will also want to confirm the proper tire size was installed and the pressure is set in all tires. I can upload the reset procedure for you if you would like.
ASE Certified Technician
Lexus Certified Master Technician
I have added a link to the procedure, just copy and paste to view the file.Let me know if you need further assistance.
http://www.mediafire.com/?sharekey=a4b1302ecc4cb87e4012e8015643d9c8962da555f0b134b6
Lexus made a change in the operating logic of the system due to the system being too sensitive causing the light to come on for no reason. I have included a link to a service bulletin, you can check to see if it applies to your LS. Unfortunately, the diagnosis of this system requires a dealer scan tool to interface with the vehicles computer. I would recommend to try the reset procedure a couple of more times and verify all four tires are set at the same pressure and are all the same size, but if the light continues to come on before the initialization process completes there may be another issue with the system and may require a visit to the dealer.
http://www.mediafire.com/?sharekey=a4b1302ecc4cb87e4012e8015643d9c80748da603d306755