You know if a sensor is bad on garage door opener by observing LED indicators, testing beam interruption, and checking response to cleaning and realignment, as bad sensors fail to maintain solid lights or cause door to not close. Safety sensors are small boxes near bottom of door tracks, one sending infrared beam and one receiving, with LEDs showing status. For LiftMaster, sending sensor typically solid yellow when powered, receiving solid green when aligned and receiving beam; if receiving LED is off, blinking, or flickering even after cleaning lenses with soft cloth and aligning sensors facing each other with clear line of sight, sensor may be bad. Signs include door opens but won't close, opener light blinks 10 times indicating sensor fault, and holding wall button down continuously closes door, confirming sensor issue. Test by waving object through beam while door closing; receiving LED should flicker and door should reverse; if no reaction, sensor bad. Physical damage like cracked lens, water intrusion, or corroded wiring indicates bad sensor. Check wiring for loose connections at sensors and opener; if LEDs remain off after tightening and cleaning, sensor likely failed. If one sensor has no light at all when opener powered, it may have no power due to bad wiring or failed sensor. Replacement sensors cost 20 to 50 dollars and can be installed DIY; if new sensors still fail, logic board may be faulty and needs professional diagnosis costing 150 to 300 dollars.
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