The system is based on 5 radio transmitters - one on every wheel as part of the valve stem - and a receiver mounted in the headliner above the 2nd row seat. In order to work, the tire pressure ECU needs to know the ID # of all 5 receivers, they all have to be sending a signal, and they all have to report a tire pressure of at least 27 psi (approximately).
Therefore, the light will blink if:
- One of the receivers is missing (for example, someone stole the spare tire)
- It thinks one of the receivers is missing because the battery is dead. (They're supposed to last 5 years, according to the service manual.)
- It thinks one of the receivers is missing, because of radio interference (I think the radio signal is in the 300-400 MHz band.)
- The ID # of one or more receivers is not recognized. (It's there, but according to the ECU, it's not supposed to be there. Maybe a wheel was changed out?)
- The temperature in one of the tires is "abnormal". (The service manual doesn't explain what that means, though. It does register a specific DTC code, though.)
If the light is blinking, the main computer has stored a diagnostic trouble code (DTC) that will help determine the specific problem. Using a Toyota handheld test tool, it's also possible to read the actual ID #'s and the pressure being reported by each tire.
If you just drove the truck off the lot and the light is on, it's possible that the dealership didn't initialize the system properly. When the truck is shipped from the factory, all transmitters are "asleep" in a pre-delivery mode to save the batteries. To wake them up, each tire needs to be deflated by at least 6 psi in a time span of 30 seconds. Then, the tires need to be refilled to a normal pressure. If that doesn't make the light go off, then the tires need to be refilled to 44 psi and the process repeated. (TSB PD007-03 - Tire Pressure Monitor Activation during Pre-Delivery Setup.)
I think this system is a lot better than the "passive" tire pressure detection system used on some other Toyota and Lexus products. The passive system uses data from the anti-lock brake system to figure out if one tire is not spinning at the same rate as the others. While that's better at handling temperature changes (because it's relative, and all 4 should be affected equally by the cold/heat), it doesn't know anything if the car isn't moving at a reasonable speed. It also can't detect 4 flats (which is rare, I admit), and it doesn't check the spare.