How does the low tire pressure indicator work?

New automobiles have an indicator light in the dash board control panel that tells when a tire is low on pressure; how does it know when the pessure is low?

newer vehicles have a on-board computer that sends impulses every few seconds to wheels. the inside of your rim below the valve stem is a small computer bank that catches the impulses then sends the info back to the o.b.c.some cars are even programed to receive each tire. left,right front and left,right rear its a more complex system so most companies only put it on the high end vehicles. in most cases the computer will indicate if one or more tires are 2lbs or more in difference pressure.....after putting all tires to same pressure, drive the vehicle about 2 miles this should auto reset the computer and tun off your light. also be aware if you mix-match tires it ma hold different PSI then the rest.