Diagnosis
The blade assembly gasket (the black rubber ring) is damaged, missing, or not seated correctly. This is the seal between the blade unit and the jar.
How to Fix It
Unplug the blender. Unscrew the blade assembly from the jar. Remove the old gasket. Clean the groove in the jar base. Press the new gasket firmly into the groove, ensuring it's fully seated and not twisted. Reassemble and tighten the blade assembly by hand—do not use tools.
If That Didn't Work
Inspect the jar itself for a hairline crack, especially around the base where the blade assembly screws in. A cracked jar cannot be sealed and must be replaced.
Heads Up
The gasket is a wear item. Replace it every 12-18 months with heavy use. Order part number for your specific jar model. Using the blender without a proper seal will damage the motor base.