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Liftmaster safety sensor lights are yellow on one side and green on the other, and will remain these colors. The yellow light will stay on as long as there is power, and the green light will remain on as long as it is correctly aligned with the yellow sensor. If the green light is flashing, flickering, or not on at all, then it is not properly aligned and the opener will not lower the door. Look closely at the green light to make sure it does not have the slightest little flicker, which indicates miss alignment. If in fact they are not aligned correctly, loosen the nut and move the sensor until the green light is constant. If this does not work, make sure the sensors are both clean of debris and there is nothing blocking the lenses. If you still cannot get the green light to stay on the sensor may be bad and and need replaced.