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I have a LiftMaster 1/2 HP from 2004 (sorry, don't know the model offhand). Just yesterday it started exhibiting this behavior:

  1. Opening the door works fine.

  2. When closing the door, it will travel 90% (door gets within 24" of ground) before reversing and blinking the lights. Subsequent attempts sometimes repeat same behavior, but often will reverse immediately or within the first 5" of travel.

  3. When the sled is detached from the door and the door is in the raised position, the sled will travel properly both down and back up 100% of the time. So this problem has nothing to do with the safety sensors or wiring.

  4. When the sled is detached and the door is in the closed position, the behavior of the sled matches #2 above. The sled will mostly immediately reverse or travel a few inches and reverse.

Notes:

  • Have tried power-cycling the unit.
  • The door is fairly well balanced and I have no trouble opening/closing it manually.

Appreciate any ideas. Thank you!

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    Stick your camera phone up there and get the exact model number. Use a 3 second timer and tape it to a stick or video mode if you have to. That will get you the manual, which will tell you what the flashing lights mean. Newer ones have overtorque sensors on the motors so the motor can "sense" when it's crushing someone. The torque is set when the door is installed and if the rails subsequently become misaligned or contaminated it can create enough additional friction to set off the overtorque. Raise the door and put a level to each end. Clean the rails. Try again.
    – K H
    Apr 22, 2021 at 4:56
  • "Fairly well balanced" isn't relevant. It is either skewed or it is not. If you can borrow a laser level that's my tool of choice to check all axes.
    – K H
    Apr 22, 2021 at 4:57
  • Could be the lube in the screw has dried out. Apr 22, 2021 at 8:53
  • In my experience, cold (I believe this somehow increases friction of the door glides) and direct sunlight on the receiver have caused similar behavior too. If the failure to close isn't constant, try checking what conditions change between working and not. Apr 22, 2021 at 13:53

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I disagree that the problem has nothing to do with the safety sensors...in particular, the optical beam which is placed near the bottom of the door.

Check and make sure there aren't cobwebs or leaves or any other crud on the emitter and sensor or hanging down from the door itself which could fool the door into thinking there is an obstruction.

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    I have to agree with this because it exhibits this behavior when the door is in the closed position but not in the up position, most likely a cobweb. Blinking light typically means something blocking the sensor. Apr 22, 2021 at 15:05
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    It could also be play in the track. If the track moves when the door descends, the safety sensor may move with it, possibly causing the OP's problem. Apr 22, 2021 at 15:41
  • Thanks for all of the help & suggestions everyone. Managed to get it fixed without having to hire a service company. I think it was a combination of Steve & Phil's diagnoses -- some cobwebs and particularly play in the track as the door descended, which caused the safety beam to flicker. I tightened the track connectors and took some slack out of the chain and gave the sensors a thorough cleaning. Thanks again!
    – Alex Scott
    May 3, 2021 at 4:13

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