Starting with the 2008 model year, and thanks to the TREAD Act passed by Congress in 2000, a Tire Pressure Monitoring System, TPMS for short, has been standard on vehicles sold in the US. TPMS uses sensors inside the wheel to continuously monitor pressure in your tires and send you a warning on the dashboard display when tire pressure falls below a designated threshold.
The sensors are mounted on the inside end of the valve stems where the air goes into your tires. Over time, these special valve stems can get damaged or become brittle with age. A TPMS valve service kit comes with the components needed to replace the factory stem, valve, and seals. Some kits also include the TPMS sensor.
The TPMS sensors in your tires communicate wirelessly with a computer in your vehicle to let you know if one or more of your tires are low on air. If the sensor goes bad, the computer will no longer be able to receive a signal and it will alert you to that fact. If, on the other hand, a valve stem fails, either because it was damaged or because it developed a leak due to age and deterioration, your tire will lose air.
If the sensor is working properly, you will see a dashboard icon light up (or see a digital readout on some models) telling you that your tire is low on air. Some loss of air in your tires over time is normal. But if a tire loses air frequently, there is either a hole in the tire, a bad seal around the bead of the tire, or a bad valve stem. A TPMS valve service kit is designed to replace a faulty TPMS valve stem.