Add air to tires without removing sensor caps. Designed for use with 507 Tow Car Sensor Pack for up to an additional 12 towed vehicle tires. Helps prevent blow-outs by constantly monitoring tire pressure and temperature. No need to remove sensors to add air. Simply connect air hose to valve built into each sensor. Comes with additional O-rings and screws. Weather resistant. No transmission when pressure is zero to extend battery life. Powered by CR1632 user-replaceable batteries. Accurate within 1.5 +/- PSI and within +/- 1°F of actual air temperature within your tires. | TST 507rv Tire Monitor System - Monitors PSI and Temperature -Flow Through System, 2-pack