The Chamberlain garage door opener(model not readily available)for my wife' s single width garage door suddenly started going back up when it was just about 18 inches from complete closure. It went back up, but the obstructio n sensor causes the opener light to flash, and the light never flashed when it did its errant going up. I checked to see if the obstruction sensor was working and it always reversed the door AND flashed the light.
I disconnected the door from the mechanism and raised and lowered it with n o signs of binding anywhere in the complete up and down travel. Then I not iced the door seemed to be going down with more force than I had expected i f it were well balanced by the two separate springs, one on each side of th e door. The door was not wet, nor were there any changes to anything that I could tell.
I had a little space between the upper end of each spring and the terminati ng end of the cable going thru the pulleys when the door was fully up, so I took the cable off and put it back on so the spring was more fully extende d as the door moved downward. I could tell a slight difference (reduction) in the propensity of the door to move down. Cycling the door repeatedly fa iled to trigger the upward motion. So, something in the opener sensed the door was too much of a downward load. Now I have to see if I can find the code name/letter on the springs so I can replace them, or at least have the springs available just in case.
Simple solution for an annoying problem.]