The original sensors are "keyed" to differrent corners of the car, i.e. the car can tell if it's a left front or right rear tire that is going low. If you have generic sensors that haven't been programmed to work with the TPMS system, the vehicle speed sensor will notice that it's above 30 and will start looking for Wheel sensor input...when it doesn't find what it's looking for you get a idiot light coming on in the dashboard.