If a new sensor was scanned by the app on first, the receiver cannot scan and says sensor was started by another device (same as the other way around), so if my phone replaces my receiver, I have to make sure I have my phone on me at all times and can charge it throughout the day. It seems either the app or receiver, not both, can scan the sensor. The very low alarms also don't ring as much as high alarms. I've slept through 50's for 3+ hours...