DIAGNOSIS The pellet sensor (a small paddle wheel in the hopper) is stuck or its mechanism is bound by pellet dust or a stray pellet.
FIX Empty the hopper. Locate the sensor—a small plastic unit on the left interior wall near the bottom. Manually spin its paddle wheel several times to ensure it moves freely. Vacuum out all dust from around it.
IF THAT DIDN'T WORK The sensor wire may be disconnected. Unplug the grill, remove the hopper access panel on the left side (a few screws), and check that the sensor's wiring harness is securely plugged into the main harness.
HEADS UP This sensor is notoriously finicky and not perfectly accurate even when working. It's best used as a rough guide. Always physically check pellet levels before a long cook.