As a note, all smoke an CO alarms have a lifespan. it depends on the unit, the ones with the sealed 10 year battery are dead when the battery dies. Most of the "regular" AA, 9v and hardwired units are 8 years and they are done, assuming they don't fail before that.
The sensitivity of the detectors starts downward after they expire. They all have mfg dates listed on the back, if it's 7+ years, toss 'em.
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