I installed a new Chamberlain B4505T garage door opener. When I press the open button the lights blink twice and nothing happens. The sensors seem to be installed correctly. There is an amber light in one and a green light in the other. If I block the beam the green light goes out. Does anyone have any idea of what could be wrong?

  • Is it the actual lights flashing and not the LED's next to the learn button?
    – JD74
    Commented Apr 3, 2023 at 15:57
  • It is the main garage lights flashing. You can hear the relay click on and off. Commented Apr 3, 2023 at 17:23

2 Answers 2


I figured out what it was. I didn't have the up and down limits set. Once I set them everything worked fine.

  • What do you mean by that? Were they not set by default?
    – isherwood
    Commented Apr 3, 2023 at 17:44
  • I don't think they can be set by default because the motor doesn't know what position the trolley is at. Commented Apr 3, 2023 at 18:22
  • Every opener I've ever installed has had some existing start and stop position which just needed shifting. I don't recall every encountering a procedure that required setup before the thing would move. Maybe it's a recent thing.
    – isherwood
    Commented Apr 3, 2023 at 18:26
  • 2
    It might be a safety feature to stop you from folding your door in half if the stop is set too low. Commented Apr 3, 2023 at 19:07
  • Had the exact same issue and setting the limits was indeed the fix. Would be a useful addition to the manual's troubleshooting section. Thanks for posting. Commented Apr 23, 2023 at 20:18

Two Flashes: A wire to a sensor is shorted out or a black/white wire is reversed.



That information is probably also in the installation manual. There's usually a troubleshooting section.

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