1. LED test: With both sensors wired, the receiver LED should be solid. If it never goes solid after careful alignment, suspect a bad sensor or a wiring issue.
  2. Wire swap: Swap the left and right sensor wires at the opener terminals. If the problem moves sides, the wiring is at fault. If it stays, the sensor may be bad.
  3. Direct run: Temporarily connect the sensors to the opener with a short new piece of 2-conductor low-voltage wire to bypass existing runs. If they work, your original wire is faulty.
  4. Distance test: Hold the sensors a few inches apart facing each other. If the LED will not go solid in this ideal setup, replace the sensors.

When replacement is needed, shop photo eye sensors. For related wiring parts (staples, low-voltage cable, connectors), see garage door parts and accessories.