Craftsman garage door opener reset?

I just changed the plastic worm gear on my 1/2 hp, 12-year-old Craftsman garage door opener, and now the motor will only spin for a couple of seconds in one direction, then a couple of seconds in the other direction. When the chain is hooked up to the sprocket, it moves the garage door perhaps a foot, but the motor operates in this way whether it's hooked up to the door or not.
When it does this, the green light on the back of the main unit flashes five times, repeatedly. I've reset the controllers and they activate the motor fine, but there's no difference whether I use the remote wireless controller or the hardwired button next to the door itself.
The sensors on the bottom of the door frame seem to be properly aligned. There's a green light on one that is on, and which turns off when I cover the beam.
I've adjusted the downward and upward pressure in every variation, and there's no difference. Similarly, I've adjusted the distance sensors on the plastic screws every which way so that the contacts touch or don't even touch, but there's no difference.
This is exactly as if the door is moving, finds an obstruction and returns to its original position, except that everything seems to be exactly as it should be. Can anyone offer a suggestion as to what I'm not doing right here?
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What model do you have? I have the manual that covers the following models: 139.18616SR; 139.18417SR; 139.18415SR; 139.18315SR
There is no Factory Reset but there are procedures for solving various problems. If your unit is one of the above, I can copy the directions for you.
Ron

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