How can I tell if my garage door sensor is bad?

You can tell if your garage door sensor is bad by observing LED indicators, testing beam interruption, and checking response to cleaning and realignment, as bad sensors fail to show solid lights or prevent door from closing even when aligned. Safety sensors near bottom of door tracks have LEDs: sending sensor typically solid yellow or orange when powered, receiving sensor solid green when aligned and receiving beam. If receiving LED is off, blinking, or flickering even after cleaning lenses and aligning sensors facing each other, sensor may be bad. Signs of bad sensor include door opens but won't close, opener light blinks 10 times indicating sensor fault, and holding wall button down continuously closes door, which confirms sensor issue. Test by cleaning lenses with soft cloth, aligning sensors until LEDs solid, and checking wiring for loose connections at sensors and opener; if LEDs remain off or blinking after these steps, sensor likely bad. Waving object through beam should cause receiving LED to flicker and door to reverse when closing; if no reaction, sensor bad. Physical damage like cracked lens, water intrusion, or corroded wiring indicates bad sensor. Sunlight interference can mimic bad sensor, so shade sensor and retest. 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.