Opener runs but chain or belt sags noticeably and may slap the rail during operation.

Safety first: Disconnect power before adjusting chain or belt tension. Keep hands clear of sprockets and pulleys.

• Locate tension adjustment: At the end of the rail opposite the opener head, find the tension bolt and nut assembly for the chain or belt.
• Adjust tension: Use a wrench to tighten the nut in small increments. For chain drives, there should be about 1/4 inch (6 mm) of sag at the midpoint. For belt drives, the belt should be snug but not overly tight.
• Inspect idler and sprocket: Check the idler pulley and drive sprocket for wobble or wear. Replace if noisy or visibly damaged.
• Test operation: Restore power and run the opener through a full cycle. Listen for reduced slapping or rattling. Do not overtighten; excessive tension can damage the motor and gears.