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Remove the cover from the air gap. Check to make sure that there is no debris obstructing the gap. Next, underneath the sink, disconnect the line that connects the dishwasher to the garbage disposal. Drain the wastewater into a bucket.
Once the line is disconnected, use a mirror to check to see if the knockout plug has been removed from the garbage disposal. It should only be in place if there is no dishwasher. To knock out the plug, first disconnect the garbage disposal; then drill a hole through the plug.
Reconnect the garbage disposal. Hook up the wastewater line that connects the dishwasher to the garbage disposal.
Test your work by running the dishwasher. With the knockout plug cleared, the waste water from the dishwasher should be draining into the garbage disposal, not through the air gap.