billie.... Max did answer your question....correctly!... the light will come on if there is a 10% +/- difference in one or more tires.... mine have done it a couple of times in the 3+ years I have had it. Fill all the tires to the same pressure and then go for a drive.... at some point it will reset. I have had it reset within a couple of miles and other times not until I drove 25 or so miles....

FYI, you can get the light on during winter if the car is outside... I usually have that happen a few times a year... after the tires warm up it goes out.