The TPMS (Tire Pressure Monitoring System) warning light is the yellow horseshoe-shaped icon on your dashboard. It illuminates when one or more tires drop 25% below the manufacturer-specified pressure.
25% below spec is the legal trigger but it's also late. By the time the light comes on, your tires have been operating in suboptimal range for days or weeks. Fuel economy has dropped. Wear has accelerated. Wet-weather grip has degraded.
Cold weather is the most common trigger. A 30°F temperature drop reduces pressure by roughly 3 PSI. If your tires were already 1–2 PSI low going into a cold snap, the light comes on overnight.
Topping up the tire usually clears the light after a few miles of driving. The TPMS needs vehicle speed to recalibrate. If the light stays on after topping up and driving, you have a different issue — maybe a slow leak in one tire or a TPMS sensor failure.
TPMS sensors have batteries that last 5–10 years. When they fail, the light stays on permanently. Replacement runs $40–$80 per sensor at a tire shop.