For vehicles with TPMS sensors, if the light blinks or is on and the pressure is verified good, the sensor is faulty or has a bad battery. My wife's car currently has this problem, the light will randomly come on for a couple days and then go off. Each time it does, I check and the pressures are good. Tires were recently replaced, so I will be returning for a new sensor when I have time. For cars like my Mazda3 that use the wheel speed sensors, there is usually a button on the dash somewhere to "zero" the calibration when you have refilled the tire.