Diagnosis
The limit adjustment knobs on the back of the motor unit are loose or the control board is faulty. The 8355W uses manual dials, not electronic settings.
How to Fix It
First, reset the limits firmly. Unplug the opener. Turn both the 'UP' and 'DOWN' limit dials fully counter-clockwise. Plug it in. Run the door down manually to the fully closed position. Now, adjust the 'DOWN' dial clockwise until the door light blinks. That's the set point.
If That Didn't Work
The potentiometers inside the dials are worn. The entire logic board may need replacement (part #41A5021). Drifting limits is a known symptom of a failing board.
Heads Up
When limits drift, the door can try to travel too far. This puts extreme stress on the trolley, belt, and door. Fix it promptly to avoid a bigger repair.