They just stop working! It’s worth bearing in mind that no appliances like hard water and does shorten their life. My non-stick plate is no longer non-stick and can’t remove the gunge no matter what I try.
Years ago I used an appliance descaler and left it overnight to soak but that didn’t work either.
My cynical self reckons that they’re deliberately designed for regular replacement.
Edit cos I just saw your edit: I used to buy ionised water, now just use water from the filter jug. Tap water not been used for, er, 20 years.
Am on my second Bosch.