I must be completely in the dark, because I didn't even know we had a tire pressure light in the car. Mine must be a broken one, because I've never seen it come on when I've had a flat tire. One of the things that can cause your tire to lose air is a nail. I know this because my girlfriend recently had a slow leak and when she finally took it in, they found a nail. At least they got to it before she was stuck on the road.