My wife loves her Keurig. I’m the one that has to maintain it, though.
When that “DESCALE” message comes up on the little screen, it’s time to run some citric acid through the machine and get rid of all those fun hard water deposits that inevitably make it through the filter. Don’t buy the overpriced Keurig brand, just grab the cheapest citric acid cleaner you can find.
However, before you dive in, you’ll need to invoke the special “Descale Mode” on the Keurig. Activate this mode by powering off the Keurig, then holding the “8oz” and “12oz” buttons until the “K” button lights up and “DESCALE” appears on the screen. Follow the directions on the bottle from here, and you’ll be in good shape! The Keurig will return to normal function once the program is finished.