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Quick answer: on K-Supreme and K-Slim machines, the descale light only turns off if the machine was actually in descale mode when you cleaned it. Enter it by holding the 8 oz and 12 oz buttons together for about 3 seconds while the machine is plugged in but powered off. Descaling without entering the mode leaves the light on no matter how clean the machine is, which is why most stuck lights happen.
The fix, in order
1. Run the descale in descale mode
Power off, hold 8 oz and 12 oz for 3 seconds until the brew button flashes and the descale light goes solid. Fill the reservoir with descaling solution (or a vinegar mix), brew repeatedly into a large mug until the add-water light comes on, then rinse with at least two full tanks of fresh water the same way. The machine exits descale mode on its own after the rinse tanks and the light clears. Do not open the lid or pull the tank mid-mode; that cancels the cycle and you start over.
2. The stuck-light reset
If you genuinely finished the full cycle and the light stayed on, try the reset with the machine powered off: hold 8 oz and 12 oz for 3 to 5 seconds. Some production runs respond to 8 oz plus 10 oz, or 8 oz plus the Strong button instead, so try all three before moving on. K-Duo owners: descale both the pod side and the carafe side, then hold 8 oz and 10 oz for 3 seconds.
3. Clean the needles
Scale-clogged entrance and exit needles can keep the sensor tripped after a clean descale. Remove the pod holder, clear the exit needle from below with a paper clip, do the same for the entrance needles, then run a water-only brew.
4. Second full cycle for heavy scale
Machines that went a year or more between descales often need two full solution cycles; the first loosens the buildup and the second clears it. A proper descaling kit costs less than a week of pods and outperforms straight vinegar on heavy scale.
5. Still on?
The machine brews fine with the light on, so you are safe while you sort it. If it survives all four steps, it is likely a stuck flow sensor, and Keurig support (1-866-901-BREW) has a strong record of replacing those even slightly out of warranty.
Keep it off
Descale every three months, sooner with hard tap water, and use filtered water between cycles. While you are maintaining things, our Keurig caffeine chart covers what is actually in those pods, and the frother fix guide handles the other half of the counter.