How can you tell if a garage door sensor is bad?

You can tell if a garage door sensor is bad by observing LED lights, door behavior, and physical condition of sensors that indicate failure rather than simple misalignment. First, check LEDs: sending sensor should show steady yellow or amber when powered, receiving sensor steady green when aligned. If either LED is completely off despite power to opener, wiring may be cut or sensor circuit board bad. If green LED blinks or flickers even after cleaning lenses and aligning sensors to face each other directly at same height, and after tightening brackets and removing obstructions, sensor may be failing. Door behavior symptom is door starts to close then immediately reverses and goes back up, opener lights flash 10 times, and remote will not close door but wall button will close only if held down continuously, indicating sensor issue; if this persists after alignment and cleaning, sensor likely bad. Physical signs include cracked lens, water damage or corrosion from moisture, especially if gutters above garage overflow and drip water onto sensors, damaged wiring chewed by animals, or sensor that feels hot. Test by swapping sensor positions or checking voltage at motor unit; if still off, sensor bad. Another test is to hold sensors face to face close together; if green becomes steady green when close but not when mounted, alignment or sunlight interference is issue, not bad sensor. If sensors are more than 15 years old, they often fail. Replacement sensors cost 20 to 50 dollars pair and professional installation 75 to 150 dollars.