The detectors have an effective life of so many years depending on the manufacturer. My MH is almost 9 yrs old and the all 3 detectors should have been replaced anywhere from 5 to 7 yrs. So you can see I waited too long. Each winter I review all the paperwork/warranties, etc and realized I had not seen the warranties on all 3 detectors so I replaced all 3 last spring. Ironically my propane leak detector activated a couple of days after I had order replacements. I checked everything for safety's sake and found the detector to be defective but no leaks. I replaced all of them before we took our big summer trips. A word to the wise so be safe y'all. :B