My LiftMaster chain-drive opener quit working suddenly. I disconnected the trolley, took off the covers and watched. The lights come on, the unit will click and the motor will spin slightly (1/8 turn or less), then the unit stops instantly. If I press the button again it clicks. Press it again and it clicks, motor again turns slightly, and stops again, virtually instantly. I tried unplugging the unit, waiting a few minutes and replugging. No change.

Door is in the down position.

Trolley has been disengaged, garage door opens and closes smoothly, springs and alignment are good.

Gears are fine, I replaced gears and the sprocket shaft bearings about a year ago, all are good.

Any ideas?