Diagnosis
A single flash indicates a problem with the safety reversing sensors. Their alignment is off or the wiring is damaged.
How to Fix It
Check the sensor alignment. The LED on each sensor (one has a green light, the other amber) must be solid, not blinking. Loosen the bracket and aim the sensors directly at each other until both lights are steady. Tighten the bracket.
If That Didn't Work
Check the wire connections at the back of the motor unit. A loose or corroded pin in the sensor terminal block (usually labeled 1 and 2) will cause this. Unplug the opener and reseat the wires.
Heads Up
Sunlight directly hitting the sensor lens can overwhelm the receiver. Shield the lenses with the included sun shades or a small piece of tubing if this happens in the afternoon.