Our CO (only not combination) detector woke us 2 nights in a row. I remember reading something about it that once it alarms it does not fully re-set. Sorry I can’t find the reference now and possibly specific to that model. We quickly replaced that unit with new and relocated it out of the furnace heat airstream. This is very different than our experience with fully resettable smoke detectors (located higher in the room) and “raw” propane detector (located near the floor).