A system for monitoring tire pressure and producing a first signal in the event that a low tire pressure condition has been sensed and a second, different signal in the event that a low tire pressure condition has not been sensed and a third signal in the event that the system is not operable. The system comprises a sensor for each tire and an associated transceiver antenna coil. Each sensor comprises a pressure switch and a circuit that has a first resonant frequency when the pressure switch is in a first state and a second, different resonant frequency when the switch is in a second state. An excited circuit associated with each transceiver antenna coil generates an AC electromagnetic field across the transceiver antenna coil and a detector circuit is operable to demodulate information communicated passively by the sensor that reflects its resonant frequency.