It is a sign that your sensors are out of alignment.
I suggest that you tweak them and make sure the green light has a steady glow.

Cold weather affects the receive unit of older Craftsman garage door sensors. You've tried tweaking the alignment. You've checked the connections. Everthing is as it was when it worked but now the weather is cold and the garage door won't close. The green light on the receiver is off. You've tried warming the sensor with a hair drier and it miraculously woke up after a few minutes of gentle heat. Well, are you ready to fix this thing or replace it? Let's fix it. Remove the clip on wire and unscrew the sensor from the bracket so you can hold it in your hand. Notice the seam that runs lengthwise along the unit. Using a sturdy blade such as a carpet cutting knife, score the seam several times being careful not to apply too much pressure or cut yourself. Carefully pry the shell into two parts. Be careful not to lose the lens when separating the pieces. Gently lift out the printed circuit board. On this board are two electrolytic capacitors. One is 470uf at 10 volts. The other is 1uf at 50 volts. (Probably only the 470uf part is bad but I replaced both capacitors because I don't like doing the same job twice). Go to Radio Shack or your favorite parts supplier and buy replacements. Solder them in observing the correct polarity. The board has a plus sign near the hole through which the positive side of the capacitor goes through. If you are unsure how to solder the parts in, find the nearest geek on the block and pay him a few bucks to do it for you. Put the two halves back together and use black electrical tape or whatever you've got (no duct tape pleeez!) to hold it together. Plug in the cord and mount it back on the bracket and tilt up and down until the light comes on.
You are in business again for about 5 bucks worth of parts and a little of your time. Alternatively, you can buy a new set of sensors for about 30 bucks.
RE: Craftsman Garage door opener

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Posted by profile341 (My Page) on Tue, Jan 6, 09 at 19:09
One last post: I took the old capacitors to work with me and put them on a cap checker. My suspicion was correct. The 1uf capacitor was still good and the 470uf capacitor was bad. So you don't need to replace both unless you really want to. Just replace the 470uf capacitor and your repair cost has now dropped to about 2 bucks worth of a part and your time.
PS: When you tape the two halves together, run the tape lengthwise along the seam, one piece on the top and and another on the bottom. The mounting screw will keep the halves together. You just want to keep the dust out.