Diagnosis
The pellet sensor is a simple paddle switch in the hopper. It's likely stuck or obstructed, or pellets are bridging above it, creating a false empty reading.
How to Fix It
Free the sensor paddle. Reach into the hopper and manually move the black plastic paddle near the bottom back and forth several times to ensure it swings freely.
If That Didn't Work
The wire connection to the sensor might be loose. Unplug the grill, remove the hopper access panel (a few screws), and check the connector on the sensor is secure.
Heads Up
The 'LO' warning is conservative by design. You still have several hours of cook time left when it first lights. It's a reminder, not an immediate shutdown.