Smoke alarms rarely fail all at once. They fail in small steps that feel like nuisances. A low-battery chirp goes off at night, someone pulls the battery to make it stop, and the alarm sits on the ceiling looking like it is still doing its job. Nobody decides to leave the house unprotected. The chirp just gets handled in the way that makes it quiet.
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What goes wrong first
The first sign is a chirp that repeats for days with no one answering it. The second is a battery that has been removed and not replaced. The third is an alarm covered with tape, a bag, or a towel to cut the noise. The fourth is an alarm that is simply old. The date of manufacture is printed on the back of most units, and the manufacturer's instructions say when to replace them. If the date is past that point, the unit should be replaced even if it still sounds when tested.
How to walk the house
Go room by room and do three things at each alarm. Press the test button and listen for a full, loud sound. Look at the back for the battery type and the manufacture date. Check that the cover is clear and the unit is still mounted and not hanging. Note any alarm that does not sound, has a battery missing, is covered, or is past its date. If the home has a carbon monoxide alarm, check that one the same way, since the same steps apply.
What the findings tell you
A chirp left going for several nights means no one is clearly responsible for answering it. A missing battery means that alarm is not protecting anyone right now. A covered alarm means the noise was treated as the problem instead of the chirp. Once you know which of these you are looking at, the question is simple: who replaces the batteries, who runs the test each month, and who answers when an alarm chirps? If no one has a clear answer, the gap is the finding.
The next move tonight
Tonight, use a stable step stool, not a chair, and press the test button on every alarm in the home. Write down any unit that does not sound, has no battery, or is past the date printed on its back. Replace the batteries in the ones that need them, put a replacement on the list for any unit that is past its date, and tell Dad which alarm you changed and which person he should call if one chirps again. For a wider look at where to start on the rest of the home, Start Here lays out the order.