DIAGNOSIS: The infrared 'ice full' sensor at the top of the ice chute is dirty or blocked. It can't detect the ice level, so the machine runs continuously.
FIX: Unplug the unit. Use a cotton swab dipped in vinegar to gently clean the two small, clear plastic sensor windows inside the ice chute. Dry them thoroughly with a second swab.
IF THAT DIDN'T WORK: Ambient light interference. These sensors can be fooled by very bright kitchen lights shining directly into the chute. Move the machine to a slightly shadier spot or angle it away from direct light.
HEADS UP: If the machine overflows for too long, the excess meltwater can overwhelm the drain and cause a leak. Address this as soon as you notice it.