TPMS stands for Tire Pressure Monitoring System, which is a device installed in a vehicles tires to monitor the air pressure and alert the driver when the tire pressure is too low or too high. The purpose of the TPMS is to warn the driver that at least one or more tires are significantly under-inflated, which could create unsafe driving conditions. The TPMS low tire pressure indicator is a yellow symbol that illuminates on the dashboard instrument panel in the shape of a tire cross-section with an exclamation point. There are two types of TPMS: indirect and direct. Indirect TPMS typically relies on wheel speed sensors that the anti-lock brake system uses, while direct TPMS uses pressure monitoring sensors within each tire that monitor specific pressure levels. The advantages of direct TPMS include delivering actual tire pressure readings from inside the tire, while the disadvantages include proprietary systems that make installation, service, and replacement confusing for consumers and auto shops, and sensors that are susceptible to damage during mounting/demounting. Under U.S. law, the TPMS is required to warn the driver of cars and light trucks when tire pressure is 25 percent above or below the recommended pressure for that vehicle. When the TPMS light comes on and stays on, at least one of the tires is at a low pressure level, and the driver should check the pressure of all the tires with a gauge and determine the cause of pressure loss and add air or service the tire(s) as appropriate.