I have a Craftsman 139.18096 opener. Worked fine for several years. Lately it has been erratic in closing. It will start to close the door and move anywhere from one inch to almost close and then re-open. The flashing light indicates an obstruction, but there is no obstruction and the green light on the receiver remains on steadily. Often it will close normally after a half hour of more wait. on an "experts" advice I changed the sensors. No improvement. I have disconnected the door and even taken the chain off the drive without success. Any advice or anyone know where I can get a schematic of the control board?