For the last several years I've had an ongoing problem with my garage door opener. Several years ago, the garage door opener that came with my house stopped working right, so I got a new one. My dad came over to help me install it (I'm not exactly Mr. Handyman). We proceeded to replace the motor only and left everything else alone. There was no need to change anything else, because the motor was the only thing that was bad.
For the last several years I've had a problem where the garage door opener refuses to close properly. It would close part way, stop, and then reverse itself and open up all the way again. At the end, the light would blink repeatedly, as if there was some kind of error. For a long time, I just forced it to go down by holding the button until it closed all the way. This really annoyed me, though because I could never close the garage door unless I was inside the garage. It makes it kind of useless.
This past weekend, I asked Dad to come by so we could swap out the old electric eyes that were left over from the previous garage door opener, with the new ones that came with it. I figured that the old electric eye wasn't working right. This would cause the eye to think something was blocking it, so it would refuse to close the door.
Dad came over and after some experimentation with the old and new electric eyes, we finally figured out what the problem was. There was nothing wrong with the old electric eye. The problem was we had connected it wrong to the motor! This was no doubt screwing up the signal causing it to not work right. It took some trial and error and reading the installation instructions (which weren't very clear), before we figured out the wiring was wrong. Finally, after fixing the wiring, the old eye worked perfectly with the newer motor. It's worked flawlessly ever since.
The moral of the story - Read the instructions! Don't assume you know how it works! :)