To my observation, the Sonicare build quality is not that great. Mine, because not wipe dry after use, or some other reasons, might have cracks at the bottom, and caused it to stop functioning after about five years or four years, I can't tell. I got a replacement on the original purchase because of severe mold problem, but the replacement has the same problem of course, so I can't tell how long I was using it. By opening the brush case, (causing irreversible damage, although glued back, no longer working properly) I found corrosion on the circuit board. After I opened the casing, it actually started to run, means the circuit came back to life. But I think when I put it back and glue to seal, the spacing between the electro-magnetic drive and the plastic housing has too big a gap, so it won't drive the header enough to perform any cleaning
OK, I'm electrical engineer, not mechanical, so I screwed it up.
On the other hand, my oral-b, bought at the same year, way cheaper, and didn't go through the replacement, so should have worked more than 5 years to now, is still in great shape. Of course, I bought a new 5000 series one from the amazon deal with 50% rebate last year, and am using that now.