Originally Posted by AlvinByars
To save battery power, the TPS is only activated when the car reaches approximately 20 mph and a roll switch is closed. Approximately every 30 seconds the TPS monitors the internal pressure of the tire.
I remember reading this in the service manual, but I wonder if this is really how they work. The only time my TPM light has ever come on was when the car sat out overnight and the temperature dropped significantly. The light came on when the car was first started and hadn't even moved. It seems that if the monitor can detect when the tire pressure has dropped while the car is sitting, it must be transmitting all the time. Maybe this was changed after the service manuals were published.