The sensor sends "heartbeat" signal to the Smart Box hourly.
If the Smart Box has not received the signal from the sensor in the "Sensor Supervise Time" then the system will issue a warning message.
At this moment please check the following items:
1, The sensor is still existing and working.
2, Replace with new batteries and press the test button to check the "dB" reading on the display.
If the reading is lower than 40dB then you should move the sensor to a better location.