Had the same problem, went off at 2 am .. Identified the problem one and changed only to have the same thing occur with another one 3-4 weeks latter changed them all and upgraded some to carbon monoxide/ smoke .. These were electric dectors tied together. As long as you are at it make sure you have a couple of the battery types if yours aren't battery backup just in case the electric is out or put out by afire.. Better to be safe then sorry !