This 4-sensor TST 507 TPMS monitors your tires' pressures and temperatures and reports remotely to a color monitor. Alerts will notify you of any readings outside of your custom high and low alert parameters. It reports up to 4 trailers/towables, and features a user replaceable battery. Flow-thru sensors allow you to air the tires without removing the sensor. Note - Flow Thru Sensors must be placed on metal valve stems, not rubber valve stems. If you have rubber valve stems then purchase the TST system with CAP sensors.