Craftsman 1/2 HP garage door won't open
I have a 2007 Craftsman 1/2HP garage door opener (Part# 41AC150-1) that worked fine until yesterday. It is one door for a double car garage. When I press the pad or the remote to operate the door, the door will move about 8 inches in one direction, stop, and then completely reverse back to its starting position. If the door is closed and I press the pad, it will open about 8 inches and then close. If the door is open and I press the pad, it will close about 8 inches and then open.
What I have tried:
Both sensors have steady lights. The one on the left is a steady amber color and the one on the right is a steady green color. Neither light blinks when I press the pad. However, the main light on the unit itself (the square amber light by the 'learn') does blink.
I have made several slight adjustments to the 'up force' and 'down force', but none of the adjustments make a difference.
I opened the back cover and checked the black plastic cap in front of the motor shaft. It was a tad bit loose and I pushed it on as far as it would go.
All of the rollers are in place and the door moves freely when I open and close it manually.
Any ideas as to how to fix the problem? Thanks!