The knockout plug
Disposals are sold for kitchens with and without dishwashers, so the dishwasher inlet on the side comes sealed by a thin plastic disc molded inside it. When a dishwasher will drain into the disposal, that disc has to be knocked out from the inside of the inlet before the hose is attached.
If it is missed, everything looks fine until the first dishwasher cycle. The disposal grinds and the sink drains normally, but the dishwasher ends its cycle with standing water in the bottom. The good news is that this is a short fix. The bad news is that someone has to disconnect the hose to do it, and the disc has to be fished out of the chamber afterward so it does not rattle around.
How the fix usually goes
Power off
The disposal is unplugged or its breaker is switched off. The dishwasher is also turned off.
Disconnect the hose
The clamp is loosened and the dishwasher hose is pulled off the inlet, with a towel ready for water.
Knock out the plug
A screwdriver and light tapping punch the disc into the grinding chamber.
Retrieve the disc
The loose plastic piece is removed from the chamber with tongs or pliers.
Reconnect and test
The hose is reattached and clamped, and a short dishwasher cycle confirms it drains.
Other causes if the plug was already out
- Kinked dishwasher drain hose behind the dishwasher or under the sink.
- Clogged air gap on the countertop, if your kitchen has one.
- Food packed into the disposal's dishwasher inlet.
- A clogged trap or kitchen drain line downstream.
- A clogged filter or drain pump inside the dishwasher itself.
Example: a weekend install
Illustrative example. Not a real job or customer.
A homeowner replaces an old disposal on Saturday. The sink drains perfectly. On Sunday the dishwasher finishes with a few inches of water in the bottom, and the countertop air gap overflows during the drain cycle.
- The disposal power is turned off and the dishwasher is not run again.
- The dishwasher hose is removed from the disposal's inlet.
- The inlet is still sealed by the knockout plug.
- The plug is knocked out, retrieved from the chamber, and the hose is reconnected.
The next dishwasher cycle drains fully, and the air gap stays dry. No new parts were needed.
What drives the cost
If the plug is the only problem, this is a quick job. If a professional did the original installation, ask whether they will come back to correct it. Cost climbs only if the cause turns out to be a damaged hose, a clogged air gap or a drain-line clog, which are separate repairs. Get any added work priced in writing before it starts.
Questions to ask
- Was the knockout plug removed, and was the loose disc taken out?
- Is the dishwasher hose routed through an air gap or a high loop?
- Did you run a dishwasher drain cycle to test?
- If the plug was out, where is the blockage?
Common questions
Can I leave the knocked-out disc inside the disposal?
It should be removed. A loose plastic disc can rattle, jam the unit or end up in the drain.
Is it safe to run the dishwasher until it is fixed?
Avoid it. The water has nowhere to go and may back up through the air gap onto the counter.
My dishwasher does not drain into the disposal. Does this apply?
No. If the dishwasher drains into a separate connection, look at its hose, filter and the drain line instead.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.