Reason 1: Reflective lens rupture.
Solution: Replace the reflective lens.
Reason 2: A and B slots on water sensor rusted or not connected good.
Solution: Check connection and assure water flowing smoothly from point A to B.
Reason 3: Optical path changing.
Solution: Re-adjust the optical path.
Reason 4: Setup in Control Panel.
Reason 5: Long working, the water temperature in the tank is too high.
Solution: Replace the cooling water.