Sensor wire shorted or reversed

When a LiftMaster or Chamberlain opener flashes 1 up-arrow and 2 down-arrows, the sensor circuit isn't open — it's shorted or wired backwards. In practice that's usually a staple pinched through the wire's insulation letting the two legs touch, or the striped and solid conductors swapped at the motor-head terminals. The door typically refuses to close and the fix lives in the wiring, not the door hardware.

This is a fiddly-but-safe DIY job on low-voltage wire with no tension parts. Check the terminals first for reversed conductors, then run your eye along the whole cable for a crushed staple or a rub-through. Because it's inexpensive bell wire, replacing an old, heavily-stapled run outright is often faster than hunting one pinch. Bring in a tech only if you'd rather not trace wire or the short hides behind a finished wall.