Troubleshooting a LiftMaster opener with a flashing light involves inspection of the safety-reverse sensing system. LiftMaster motors will not operate when something obstructs the infrared beam between the safety sensors at the bottom of the garage door tracks. An open door will not close if the infrared beam is broken. The main light will blink 10 times, indicating a problem. The LED on the motor will also blink and serves as a diagnostic tool for troubleshooting.
Place a step ladder under the LiftMaster motor so you can comfortably access the "Learn" programming button. Some LiftMaster motors have the button hidden behind the lens cover. Press the tabs of the lens cover with your fingers, and allow the cover to hang by its lower tabs.
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Count the number of times the LED light beside the "Learn" button flashes before it pauses and begins blinking again.
Go to the safety sensors, and follow each of the wires where they staple to the wall and ceiling, if the LED blinks one time and pauses. This is an indication of an open wire, usually due to a staple. Look at each staple that secures the wire to the wall or ceiling. Pull out staples that appear to go through the wire instead of around it with a pair of pliers.
Check the wire connections at the motor, if the LED blinks twice before pausing. This is an indication that the black and white wires are reversed in the quick-connect or there is a short in the low-voltage wire. The white wires for both sensors should be together in one quick-connect terminal and the black wires should be together in another quick-connect terminal. If the wires are connected correctly, there is a short in the wire, requiring replacement of the low-voltage wire.
Follow the door push button wire and look for a staple inadvertently piercing the wire, if the LED blinks three times. This is an indication of a short door control wire or bad door control. If the wires appear good, remove the screw securing the door control to the wall with a Phillips-head screwdriver. Loosen the two terminal screws on the back, and pull the two wires off the terminals. Touch the wires together. If the door operates, replace the door control. If the door does not operate, replace the door control wires.
Clean the safety sensor lenses and realign them, if the LED blinks four times before pausing. This is an indication of misaligned sensors. Loosen the wing nut securing the sensors and realign until the lights on the sensors illuminate. Tighten the wing nuts once sensors are aligned.
Contact a door service professional if the LED blinks five or six times. This usually involves replacing a sensor or the logic board of the unit.