Pretty sure it's an RF (Radio Frequency) transmission, like what your key FOB transmits when you hit a button to lock or unlock a door. I think the signal is much weaker to conserve on battery power, just strong enough for a sensor near the tire to pickup. I'm sure a google search would yield a more complete answer if you want to know more...

 

Not sure about this. During service at the dealership, I had them check the tire pressure. The guy used the same device they connect into the OBD-II port to check the pressure at each wheel. Don't know how much battery saving is going on. Based on that, I think it's always running.

 

Makes sense... I would believe that

 

2013 Nissan Altima Owners Manual In case of emergency 6-3

TIRE PRESSURE MONITORING SYSTEM (TPMS)

This vehicle is equipped with the Tire Pressure Monitoring System (TPMS). It monitors tire pressure of all tires except the spare. When the low tire pressure warning light is lit, and the “Tire Pressure Low - Add Air” warning appears in the vehicle information display, one or more of your tires is significantly under-inflated. If equipped, the system also displays pressure of all tires (except the spare tire) on the display screen by sending a signal from a sensor that is installed in each wheel. If the vehicle is being driven with low tire pressure, the TPMS will activate and warn you of it by the low tire pressure warning light. This system will activate only when the vehicle is driven at speeds above 16 MPH (25 km/h). For more details, refer to “Warning/indicator lights and audible reminders” in the “Instruments and controls” section and “Tire Pressure Monitoring System (TPMS)” in the “Starting and driving” section.

 

The RF sending unit is installed into the valve stem.

 

The system uses RF transmitters located in each tire, each transmitter has it's own unique RF ID number.

The system utilizes 3 antennas located on the vehicle to receive the data from the transmitters.

The system uses a triangulation formula to determine the position of each transmitter and it reported pressure to the TPMS system.

 

Very concise, simple, and clear description of how the system works, thanks!