Why the sensing element wears out
A carbon monoxide alarm has two jobs: detect the gas and sound a warning. In many household models, an electrochemical sensor does the detecting. Carbon monoxide reaching the sensor triggers a chemical reaction that produces a small electrical signal; the alarm evaluates that signal and sounds when it reaches a danger threshold.
That sensing material does not stay equally responsive forever. Exposure to ordinary air and gradual changes inside the sensor can reduce its ability to produce a reliable signal. The detector’s expiration date marks the end of the period its manufacturer expects the sensor to work reliably—not a sudden moment when it is guaranteed to fail.
Why the test button is not enough
Pressing the test button checks that the alarm’s circuitry, speaker, and power source can respond. It does not necessarily confirm that the sensor can still detect carbon monoxide accurately. A loud test sound is therefore not proof that an old detector remains protective.
Check the replacement date printed on the alarm or in its instructions, since service life varies by model. When that date arrives, replace the whole unit unless the manufacturer specifies a different procedure. An alarm can still make noise while its ability to sense the hazard has weakened.
