Could be a bad antenna on the circuit board or some type of interference with the signal may want to throw some new batteries in the remote and see if that fixes it.

Other wise you can use a small gauged wire and extend the antenna out toward the front of the garage to help pick up the signal when the door is closed. Or replace the circuit board.

You may have interference from some other wireless device (i.e. cordless phone) in the vicinity. You could try unplugging them one at a time to see if the GDO will perform better.

Call Sears. You have a warranty.

I have a Sears 1/2 HP Garage Door Opener that is very old. (AUG 1 4 1981)
After lot of trying this is what I see, when I bring the recieving sensor to within 4 feet of the sending sensor then the Garage Door is close or open , everything works good. But if I'm goin farther nothing hapend.
I am looking for help on how to fix this problem, as cheaply as possible of course :-). Is there any thing I can do ? Or is there some thing else I am not doing correctly? Any help will be appreciated.

Thanks.

If your opener is a 1981 model there won't even be parts available. You might check into a universal receiver and remote that will work. They are available at Sears, Radio Shack and I believe Home Depot.