DIAGNOSIS The float sensor in the reservoir base is stuck in the 'low' position, or there's mineral buildup on the sensor contacts.
FIX Remove the water reservoir. Turn it upside down and locate the small rectangular float chamber on the bottom. Shake it firmly or tap it against your palm to dislodge any stuck float. Rinse the entire reservoir, especially the bottom, under warm water. Dry the metal sensor contacts on the bottom of the reservoir and inside the brewer with a clean cloth before reinserting.
IF THAT DIDN'T WORK The issue may be in the brewer's base contacts. Unplug the machine. Use a cotton swab dipped in white vinegar to gently clean the three metal pins inside the brewer where the reservoir sits. Let dry for 10 minutes before plugging back in.
HEADS UP This sensor is notoriously finicky. Ensure the reservoir is seated perfectly straight and pushed all the way back. Using filtered water can reduce mineral deposits that cause this false sensor trigger.