If your tire pressure is correct all around the vehicle, then you've either got a bad TPMS sensor(s), or the battery died in one or more of the sensors. TPMS sensors have batteries similar to a watch. Most last 6-8 years, while others last a very long time. Find a good tire shop near you (the Lexus dealer will suck the life out of your wallet. They should have a handheld TPMS computer and (for a fee likely) they can scan your sensors and determine the air pressure at each sensor, if the sensor has an output signal, and how much battery strength still exists. Then they can either reset your sensors if they're still viable or they can replace the dead sensors. Here in the Nashville area, the best tire shop in town charges $30 to reset the sensors in the TPMS system and $60 to replace each bad sensor. Don't think you can go grab a cheap off brand sensor and take it to the tire shop to install. Most shops will only install their chosen brand of sensor. If you buy a sensor, the reset is usually included. Hope this helps. Good luck!
