All new garbage disposals have a knockout plug. This plug restricts the flow of water between the disposal and the dishwasher. Those who do not want to connect the disposal to the dishwasher don't need to do anything with the knockout plug. But if you want the garbage disposal to work with the dishwasher, you need to remove the knockout plug. If you do not remove the knockout plug, your dishwasher will not be able to drain.
Turn the water valve under the sink off by hand. Unscrew the garbage disposal from the base of the sink by turning the disposal counterclockwise.
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Unscrew the hose clamp from the inlet where the hose connects to the disposal and remove the hose.
Lay the disposal on its side, with the inlet facing up.
Insert a flathead screwdriver into the inlet. The tip of the screwdriver should be on the very edge of the knockout plug, which is just inside the inlet.
Tap the end of the screwdriver as many times as necessary until the knockout plug comes loose.
Turn the disposal upside down to remove the knockout plug. Reconnect the disposal to the sink and the drain hose. The dishwasher now will drain.