Chain-driven garage door openers should have some slack in the chain for optimal performance. Over time, though, this slack can increase or the gear and sprocket assembly can wear, and the chain may pop off the sprockets. You can put the chain back on your automatic garage door opener using a simple adjustment mechanism. The chain connects to a threaded anchor that you can move in and out to tighten or loosen the chain as needed. Because chains are lubricated with grease, you can expect to get a little dirty while putting the chain back on your opener.
Disengage the garage door from the opener by pulling down on the emergency pull cord to release the door trolley from the opener carriage. Close the garage door manually, if it's not already closed.
Press the wall control button or remote so that the opener completes its CLOSE cycle, and the sprocket on the top of the motor unit stops spinning. On many units, the sprocket spins clockwise (when viewed from above) to close the door, but this operation varies by model. Move the chain and carriage manually so the carriage is at the door end of the opener rail.
Place a stepladder under the motor unit and unplug the opener power cord from the outlet in the ceiling. Locate the chain sprocket on top of the motor head and, if applicable, pry the cover off the sprocket with a flathead screwdriver.
Move to the door end of the track and locate the threaded, bolt-like anchor that runs through the opener carriage and connects to one end of the chain. There should be one or two nuts at each side of the anchor. Loosen the nut(s) facing the door five full turns with a 7/16-inch wrench. This will loosen the chain and give you more slack to place the chain back over the sprockets. Turn the nut(s) facing the opener motor one half-turn clockwise by hand.
Sight down the length of the opener rail to ensure that the chain has no twists. The sides of the chain without openings in the links should face up and down (parallel to the floor). You may need to rotate the side of the chain left or right to remove any twists.
Place the chain around the front sprocket on the rail. Some openers use a smooth sprocket rather than one with teeth. Move the ladder to the opener motor and place the chain around the sprocket on top of the motor unit. Move the ladder back under the carriage assembly.
Grasp the chain 1 foot from where it connects to the end of the threaded chain anchor. Pull the chain toward the garage door and hold it in position. Thread the nut(s) facing the door clockwise until it meets the bracket, then release the chain. Press down on the chain with your finger. You should have 1/2 inch of "play" in the chain. Tighten or loosen the chain as needed by adjusting the nuts. Be careful not to overtighten the chain, which unnecessarily taxes the motor and chain assembly.
Reinstall the sprocket cover, if applicable, and plug the opener power cord into the ceiling outlet. Open the garage door by hand until the door arm engages with the opener carriage. Push the wall control button to test the opener operation. Readjust the opener's limit switches, weight sensors, and safety reverse system as needed.