I have 6 of these in various rooms. They are all wired to AC and therefore should have a constant green LED lit. However 2 of them were blinking green. Also, occasionally some of the 6 sound a single beep...not sure which ones were doing it. I called tech support who said that the 2 were apparently "latched" and they were the ones that were beeping sometimes. The manual describes what latching means but doesn't say how to unlatch them. Her instructions were: unscrew the detector from its base and remove the AC connector, swing out the battery drawer to disconnect the battery, press and hold the Test/Silence button for 20 sec., close the battery drawer, press the Test/Silence button and it should alarm (3 beeps). Reconnect the AC power and the green LED should stay on constant. Finally, reconnect the detector to its base.