TPMS stands for Tire Pressure Monitoring System, which is a system installed in a vehicles tires to monitor the air pressure and temperature of the tires. The purpose of TPMS is to alert the driver when tire pressure is too low or too high, which could create unsafe driving conditions. TPMS sensors come in two varieties: direct and indirect.

• Direct TPMS: This type of TPMS uses pressure monitoring sensors within each tire that monitor specific pressure levels. These sensors are located inside the tire and take active pressure readings. They have an on-board battery that provides power to the sensor and transmitter and sends out a radio frequency to sensors on the vehicle. The data is sent to the vehicles main computer where it can use that data to determine if the tire pressure is too low or too high.

• Indirect TPMS: This type of TPMS typically relies on wheel speed sensors that the anti-lock brake system uses. These sensors measure the rate of revolution each wheel is making and can be used by on-board computer systems to compare with each other and to other vehicle operation data such as speed. If a tire is underinflated, it will spin at a different rate than normal, and the vehicles computer will notice that and will illuminate the TPMS warning light.

When the TPMS warning light comes on and stays on, at least one of the tires is at a low pressure level. The driver should check the pressure of all of the tires with a gauge and determine the cause of pressure loss and add air or service the tire(s) as appropriate. Its important to note that TPMS does not replace routine tire pressure maintenance. Its a tool that can help alert the driver when pressure is low, but a tire may drop below proper inflation long before the TPMS warning light comes on.