How to tell if your garage door sensor is bad?

A garage door sensor is bad if the LED stays off when power is connected, if both sensors are perfectly aligned and clean but the green light still will not stay solid, or if the door reverses immediately every time you try to close it even with a clear opening. Test power first by checking that wires are landed on the correct opener terminals and not pinched. Swap the sensor lenses or wiring left-to-right to isolate which side has the problem, since the failed unit will move with the swap. A consistently dim or flickering LED, water inside the housing, or melted plastic from sun exposure are also signs the sensor is failed. Replacement sensor pairs cost around 35 to 60 dollars.