We've been repairing appliances for over 50 years, and if I had a dollar for every time a customer called in a panic because their fairly new dishwasher suddenly stopped draining… I could buy a pretty nice truck.
Nine times out of ten, the conversation goes exactly like this:
Customer: “My dishwasher is only a year old and it’s full of dirty water! It ran fine for months and now it just stopped draining. I’m scared the pump died!”
Me: “Did you recently replace your garbage disposal?”
Customer: (long pause) “…Yes. About three weeks ago.”
Me: “Did the installer knock out the little plastic plug inside the disposal where the dishwasher drain hose connects?”
Customer: (even longer pause) “…There’s a plug?”
And that, my friends, is the single most common reason a dishwasher mysteriously stops draining after a new garbage disposal is installed.
What’s this “knock-out plug” anyway?
When garbage disposals leave the factory, they have a small plastic disc (a knock-out plug) molded into the dishwasher inlet port. It’s there so people who don’t have a dishwasher can use the disposal without leaking water all over the cabinet.
If you have a dishwasher, that plug absolutely, positively has to be removed before you connect the drain hose. If it isn’t, the hose has nowhere for water to go, so it just sits in the bottom of your dishwasher looking (and smelling) disgusting.
The crazy part? The plug is usually hidden inside the disposal’s inlet nipple. You can’t see it from the outside once the hose is connected. That’s why even professional plumbers sometimes forget to knock it out—especially if they’re in a hurry or it’s a replacement disposal and they assume the old one already had it removed.
Symptoms of the Forgotten Knock-Out Plug
- Dishwasher runs normally but leaves 1–3 inches of standing water at the end of the cycle
- No strange noises (the pump is spinning, it just has nowhere to push the water)
- Problem started right after a new garbage disposal was installed
- Pulling the drain hose off the disposal reveals a perfectly clean, unblemished plastic circle staring back at you (that’s the plug)
How to Fix It (Takes Literally 5 Minutes)
Tools you’ll need:
- Hammer
- Long screwdriver or punch
- Towels (lots of towels)
- A small bucket or pan
Steps:
- Turn off power to the dishwasher at the breaker (safety first!).
- Pull the dishwasher out far enough to access the back (or crawl under the sink if you’re flexible).
- Put towels and a bucket under the disposal—water will come out when you disconnect the hose.
- Loosen the hose clamp and pull the dishwasher drain hose off the disposal’s dishwasher inlet nipple.
- Look inside the nipple. If you see a solid plastic disc, congratulations—you found the culprit.
- Stick a long screwdriver or punch into the nipple and give the plastic plug a firm tap with a hammer. It should pop right out into the disposal chamber.
- Reach into the disposal (with power still off!) and fish out the plastic piece so it doesn’t get ground up later.
- Reconnect the hose securely, push the dishwasher back in, and run a quick cycle.
Pro Tips to Avoid This Nightmare
- If you’re installing the disposal yourself: Knock out the plug BEFORE you install the disposal. It’s way easier when it’s sitting on your workbench.
- If you hire someone: Stand there and watch them do it, or at least ask “Did you remember to knock out the dishwasher plug?” You’ll either save them embarrassment or save yourself a service call.
- Some newer disposals (especially Insinkerator models) now ship with the plug already removed or with a bright warning sticker. Progress!
Final Thought
So if your dishwasher suddenly won’t drain and you’ve recently changed the garbage disposal—before you call a repairman, check for the plug. You’ll save yourself $150 and feel like a plumbing genius.
