Garage door opener chain is sagging excessively or jumping on the sprocket.

Safety first: Unplug the opener before adjusting chain tension. Keep fingers away from the sprocket and chain.

• Adjust chain tension: Locate the chain tension nut on the Craftsman trolley. Tighten until there is about 1/4 inch sag at the midpoint of the chain.
• Inspect sprocket: Check the top sprocket for sharp or hooked teeth. Replace the sprocket or gear and sprocket kit if worn.
• Check chain wear: If the chain has stiff or elongated links, replace it with a Craftsman-compatible chain.
• Secure rail: Tighten all rail mounting bolts to prevent movement that can cause chain jumping.