The pressure sensor is a part of the valve stem. There's a picture of it in
this thread.
The sensors are rumored to be $100 to $150 a piece. That's a pretty expensive screw-up, if that what it is.
First check that all 5 tires are above 27 PSI - that's the level at which the light will come on for low pressure.
Beyond that, a Toyota dealer could tell you what's wrong with it by plugging in their fancy OBD scan tool.