DIAGNOSIS: That's the built-in 2-minute timer. A flashing green light signals you've brushed for the dentist-recommended two minutes. It's a feature, not an error.
FIX: No fix needed. Simply stop brushing. The brush will continue to run if you keep pressing the button, but the light will keep flashing to indicate the session is complete.
IF THAT DIDN'T WORK: If the light flashes immediately when you turn it on, or flashes in a different pattern (like with red), that's a different issue. The standard post-brush flash is a steady, slow pulse of the green light alone.
HEADS UP: Don't ignore this signal. The Pro 1000 also has a 30-second pulse to remind you to switch quadrants. Use these cues for a more efficient, thorough clean.