Here is a copy of Garage Doors For Dummies 101:
Auto reverse, and failure to respond to close command, are emergency safety functions. This only happens when the opener thinks someone or something it trapped under the door, or standing in the path of the door while you are trying to close it. There are 3 possible causes.
The 1st& most common way the safety system is fooled into thinking it should not close is misalignment of the safety sensors. They are easily bumped and knocked out of alignment. The safety sensors are located at the bottom of, and mounted to, each side of the track. They must be pointed AT each other with nothing blocking either of the sensors. When properly aligned, you should have a small light illuminated on each sensor. If they are in good alignment, but not lit, check the delicate wires for continuity and firm terminal connections. Check for loose wire connections to the control board.
When this problem occurs the remotes will not work, the door will open, but the door will stop & reverse immediately after activating the wall button down command, or if you are holding the wall button down, which bypasses the safety system, immediately upon release of the button. However, IF the door travels partially or all the way closed and then reverses, look to the 2nd & 3rdpossible causes.
The 2nd cause is the closing force, which is also part of the safety system. It could be set too light and the normal tension of the closed position exceeds what the door believes to be allowable tension. . There should be a dial on the side of your opener that has a downward pointing arrow beside it. Turn this dial slightly, in the direction indicated on the opener, to increase the closing force of the door. If opening is the problem look for the up force dial. Binding of the spring coils on the shaft is a common cause of increased tension. If the shaft is dry, oiling or greasing the shaft will restore normal tension. DO NOT use WD-40! For more on springs see:
The 3rd cause is a malfunctioning travel limit switch. This fails to tell the door it is time to stop, and the opener tries to drive the door into the ground- again exceeding what the door believes to be allowable tension. The travel limit switch-mounting setscrew can vibrate loose allowing it to move or fall off. It's trip finger can break, or the switch can simply fail. The travel limit switch is mounted to the rail about where the trolley is located in the closed-door position.
First take a quick look to see if the travel limit switch is dangling by it's wire. Next confirm the sensors are lit & pointing at each other. If the sensors are aligned, get a stepladder and do a close inspection of the travel limit switch. If it has slipped & moved you will see a mark on the rail where the setscrew was engaged. Reset the switch and tighten the setscrew.
This is caused by cold solder joints on the control board, dusty sensors exposed to direct sunlight. wire breaks making contact during expansion & open during contraction, contacts & connectors between the receiver board and the logic board responding to expansion/contraction/vibration, a disconnected or misdirected antenna , low battery power to remotes, or foil reflecting/blocking remote signals(insulate door with foam/foil.