Short definition
A knock-out plug is the factory plug sealing the dishwasher inlet on a brand-new garbage disposer. If you’re connecting a dishwasher to the disposer, you have to knock the plug out first or the dishwasher can’t drain. It’s the single most common gotcha when installing a new disposer — the dishwasher fills up, won’t drain, and the homeowner blames the dishwasher when the real problem is a sealed inlet under the sink.
What it is
Garbage disposers ship from the factory with a small plastic or metal plug pressed into the dishwasher inlet on the side of the unit. The plug is there because not every installation includes a dishwasher — when only the sink drains into the disposer, the inlet stays sealed.
When you connect a dishwasher drain hose to the disposer, you have to remove the plug first:
- Set the disposer on its side or hold it inverted.
- Insert a flat screwdriver into the dishwasher inlet from inside the disposer chamber.
- Tap the screwdriver with a hammer to break the plug loose.
- Tip the disposer over and fish the freed plug out of the chamber.
That last step matters. If the plug stays loose inside the disposer, the next time the disposer runs the impeller can throw it, jam the motor, or shoot it into the discharge tube.
Why it matters to a homeowner
Two failure modes, both DIY-fixable:
- Forgot to knock it out. Dishwasher won’t drain, water backs up into the tub. Disconnect the dishwasher drain hose at the disposer, knock out the plug per the steps above, reconnect.
- Knocked out but not fished out. Disposer rattles, jams, or won’t grind. Disconnect power at the breaker, look down into the disposer with a flashlight, retrieve the plug with pliers or a wet/dry vac.
This is a 5-minute fix once you know what’s wrong. The frustrating part is the diagnosis — most homeowners spend an hour on the dishwasher before checking the disposer.
When you’ll encounter this term
- Installing a new garbage disposer with an existing dishwasher
- Installing a new dishwasher with an existing disposer
- Replacing a disposer where the previous install never had a dishwasher