When the remote fails to operate the door opener, the first thing to check is the battery. Replacing the battery in a Sears 315 garage door opener requires a new lithium battery. Unlike other remotes, the battery in the 315MHz models uses a battery similar to a watch battery.
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Insert a small flat-head screwdriver in the seam between the front and back cover of the remote control. Twist the handle of the screwdriver and pry the cover apart.
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Pull the old battery out of the top cover. Insert a new battery with the positive side of the battery pointing to the top cover.
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Place the two covers back together and press them with your fingers until they lock.