Garbage Disposal Dishwasher Connection — Knockout Plug Guide
Every new garbage disposal ships with a knockout plug blocking the dishwasher inlet port. If you have a dishwasher, you must remove this plug before installation. Forgetting it’s the single most common mistake during disposal replacement — and it causes the dishwasher to stop draining completely.
The fix takes about two minutes. Here’s how to check, remove the plug, and connect the dishwasher drain hose properly.
Quick answer
Dishwasher stopped draining after a new disposal install? The knockout plug is still in. Look at the side of the disposal body near the top — there’s an inlet port about 1 inch in diameter. If it’s blocked by a plastic or metal disc, that’s the knockout plug. Remove it before connecting the dishwasher drain hose.
This ranks as the fourth most common DIY disposal mistake, according to plumbing community forums. We see this question constantly from homeowners who just finished an otherwise perfect garbage disposal installation.
What Is the Knockout Plug?
The knockout plug is a factory-installed disc (plastic or thin metal) that seals the dishwasher inlet port on the side of the disposal. It exists because not every household has a dishwasher. For homes without one, the plug keeps the port sealed so water doesn’t leak out during disposal use.
The inlet port sits on the side of the disposal body, typically 1-2 inches from the top. All major brands — InSinkErator, Waste King, and Moen — include this port and plug. The design is identical across manufacturers.
If you don’t have a dishwasher, leave the plug in place. It’s doing its job.
How to Remove the Knockout Plug
Remove the plug BEFORE mounting the disposal to the sink. Working on it after installation is possible but much harder because of the tight space under the sink.
First, hold the disposal on a stable surface with the inlet port facing you. Next, place a flathead screwdriver or wooden dowel against the knockout plug from inside the grinding chamber. Tap firmly with a hammer, just one or two solid hits will do, the plug should pop inward and fall into the chamber. Then, reach inside and remove the loose plug; this step is critical as leaving it in could jam the disposal on first use. After that, check that the port is fully open by looking through it, you should see straight through to daylight or the opposite wall of the chamber.
The procedure is the same across InSinkErator, Waste King, and Moen units. InSinkErator’s installation support documentation includes this step explicitly.
Video guide
Video: “How to Replace a Garbage Disposal - Easy, Step-by-Step” by LRN2DIY (knockout plug removal at 7:20)
Connecting the dishwasher drain hose
After the knockout plug is out and the disposal is mounted:
First, slide the dishwasher drain hose onto the inlet port; standard hoses with a 7/8-inch inner diameter fit all major brands. Next, secure it with a screw-type hose clamp, tightening until snug but avoiding excessive force that could crack the port. Then, create a high loop with the hose under the countertop, routing the hose upward as far as possible before dropping it down to the disposal inlet; this setup prevents dirty water from flowing back into the dishwasher. After that, check local codes since some jurisdictions require an air gap fitting mounted on the countertop instead of a high loop, refer to the Uniform Plumbing Code for details. Finally, when in doubt, a high loop will work for most installations.
Verify the flange and mounting connections are tight before testing the dishwasher drain.
Troubleshooting dishwasher drain problems
Dishwasher not draining after new disposal
The knockout plug is still in. This is the cause in the vast majority of cases. Disconnect the hose, check the port, and remove the plug if present. You may need to partially unmount the disposal to access the port from the inside.
Water backing up from disposal into dishwasher
The clog is downstream, not at the disposal-dishwasher connection. The drain hose or P-trap is blocked. Try running the disposal with plenty of cold water first. If the disposal drains fine but the dishwasher backs up, the issue is in the dishwasher drain hose itself or the high loop is missing (allowing backflow). See our clogged garbage disposal guide for drain-clearing steps.
Leaking at the hose clamp
Tighten the hose clamp one-quarter turn. If it still drips, the hose may be too worn or the wrong diameter. Replace the hose with a standard 7/8-inch dishwasher drain hose ($8-$15 at hardware stores). If the disposal inlet port itself is cracked, the port can’t be repaired — you would need a new disposal.
Dishwasher drain hose keeps slipping off
The hose diameter is wrong, or the clamp is too loose. Standard dishwasher drain hoses are 7/8-inch inner diameter. If your hose is a different size, use a step-down adapter or replace the hose. Double-clamping (two hose clamps side by side) also works for stubborn connections.
If the disposal leaks from the side at the inlet port itself, the port may be damaged and the unit needs replacement.
FAQ
What happens if you don’t remove the knockout plug?
The dishwasher has no path to drain. Water backs up inside the dishwasher, pools on the door seal, and eventually leaks onto the floor. The dishwasher may also throw an error code for a drain failure. The fix is straightforward: disconnect the drain hose, access the knockout plug, remove it, and reconnect.
Can you remove the knockout plug after the disposal is installed?
Yes, but it’s harder. Place a screwdriver against the plug from inside the grinding chamber (through the drain opening from above) and tap with a hammer. The plug pops inward. Fish it out through the drain opening with needle-nose pliers or tongs. Removing it before installation is much easier and takes 30 seconds.
Do all garbage disposals have a dishwasher connection?
Yes. All major residential garbage disposals from InSinkErator, Waste King, and Moen include a dishwasher inlet port with a knockout plug. The port location and size are consistent across brands. This is a standard feature, not an add-on.
What size is the dishwasher drain hose?
A 7/8-inch inner diameter drain hose suits most garbage disposals; ensure yours matches this gauge to avoid issues. Most hardware stores stock replacement hoses for $8-$15. Verify your current hose’s dimensions before buying, especially if it’s a different size than the standard, using an adapter or switching to the correct size prevents future problems.
Related guides
- air gap troubleshooting, California code requires this, other states recommend it
- P-trap plumbing, connection downstream of the disposal
- double-sink setup, sharing a drain with a second basin