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Hi my name isXXXXX have been an appliance service tech since 1992.Sounds like to me the person who installed the disposal did not remove the knock out plug from the drain.I would remove the drain hose from the dishwasher to the disposal, and remove the plug, then reattach the hose. Or get the person who installed the disposal to come back out, this would be there fault. This should fix the issue.