The two most common reasons a smoke alarm malfunctions (before it is a decade old and should be replaced anyway) are dust that has collected inside and nuisance triggers, such as cooking fumes or bathroom steam, says Judy Comoletti, of the National Fire Protection Association. Clean the unit first: Just remove the cover and vacuum it out. Replace the cover and press the test button to ensure that it's working. If the problem persists, relocate the unit away from steam or smoke sources. And always cover it with plastic when you do renovations, to prevent more dust buildup.