The Mighty Mule series of gate openers has a fixed limit switch for the open position. If the limit switch breaks then the piston will continue to retract until it bottoms out inside the motor housing. This will cause the control board of the gate opener to react as if it has hit an obstacle and obstructed. It will then reverse directions to back up off the obstacle. If this is happening the Rev Counter board will need to be replaced. This contains the limit switch on it. This is a fairly easy part to replace and it is inside the operator arm. No special tools are need, just a screw driver. http://www.gatecrafters.com/product_detail.aspx?productid=7382