How Long Carbon Monoxide Alarms Last and When to Replace One

A carbon monoxide alarm has a service life, and it is shorter than most people assume. The sensor ages whether or not it ever detects anything, so the clock runs from the date the unit was made, not from the day you started using it.

The date that matters is on the back

Turn the unit over. Manufacturers print a manufacture date, and many print a replace-by date next to it. That second date is the one to write in a calendar. A unit bought on sale can already have spent a year in a warehouse, which is why the manufacture date is worth a look before you buy as well as after.

The published service life differs by model, and the manual states it. What does not differ is the direction of travel: an electrochemical CO sensor degrades with time and exposure to ordinary household air, and once it is past its rated life nobody, including the manufacturer, will tell you what it still detects.

Alarms announce their own expiry

Manufacturers build an end-of-life signal into the unit precisely because the date on the back gets painted over, covered by a sticker or simply forgotten. The signal is a chirp pattern, and the giveaway is that a fresh battery does not stop it. If you have changed the battery and the chirping resumes within seconds or minutes, the unit has finished, not the battery. What each pattern means covers how to tell an expiry chirp from a low-battery chirp and from a real alarm.

Testing tells you less than people think

The test button checks the horn, the circuit and the battery. It does not expose the sensor to carbon monoxide, so it cannot tell you the sensor still works. This is the single most useful thing to know about maintaining an alarm: monthly testing is worth doing, and it is not evidence that an old unit is still protecting anybody.

The same logic applies to the display on a plug-in unit. A steady zero on the screen is what a working alarm shows in clean air, and it is also what a dead sensor shows.

Battery-only or plug-in: what it changes for lifespan

• Battery-only sealed units run for their rated life on one internal battery and are replaced whole. Nothing to remember except the date.
• Battery-only with a replaceable battery needs a battery on a schedule and the whole unit at end of life. Two dates instead of one.
• Plug-in with battery backup keeps its display and its sensor powered from the mains, and the backup battery covers power cuts. The sensor still expires on the same kind of schedule.

Whichever you have, replacing it is not an upgrade decision so much as a maintenance one. If you are at that point anyway, the carbon monoxide alarm collection compares the battery-only design against a plug-in unit with a parts-per-million display, which is the choice worth thinking about when the old one goes in the bin.

A practical routine

• Note the replace-by date on every alarm in the house, once, in whatever calendar you actually read.
• Test monthly, knowing the test covers the horn and the circuit.
• Replace the whole unit at its stated end of life, not when it fails.
• Treat a chirp that survives a fresh battery as the unit's own retirement notice.