most impact monitor systems i know of have a sensitivity setting, maybe it's set too sensitive? or is it an actual fault/error?
if so and you've replaced the sensor twice and it still faults out then maybe it's either the cable to it or the main box is just bad.
i would say call crown but i doubt they would help you, i know them and they are very tight with info.
But just knowing these systems like i do know, they are ALL troublesome in one way or the other.