Quick Answer:
If the dishwasher is draining into the sink after you swapped the garbage disposal, the most common cause is the disposal’s dishwasher inlet still has its knockout plug in place or the dishwasher drain hose at the disposal is loose or kinked. Turn off power to the disposal, check the inlet for the knockout plug, and confirm the hose clamp is tight and the hose runs straight. If you’re not comfortable with these checks, a pro is a good choice.
Why This Happens
Most new or replacement disposals ship with a small metal knockout plug covering the dishwasher inlet. If that plug isn’t removed before connecting the dishwasher drain hose, the dishwasher has nowhere to go but back into the sink. A loose clamp or a kinked hose at the disposal inlet will also let water flow the wrong way. Other causes include a clogged sink trap or a stuck or clogged disposal that won’t allow water to pass.
For related issues, see the brief guides on Sink fills only on hot cycles and Sink backs up after installing dishwasher for different but sometimes overlapping causes.
Step-by-Step What to Do
Step 1 — Stop equipment and cut power
- Shut off the dishwasher and stop using the sink.
- Turn off power to the disposal at the circuit breaker or unplug the unit. Never work on the disposal with power connected.
Step 2 — Inspect the dishwasher drain hose at the disposal
- Look where the dishwasher hose connects to the side of the disposal. Confirm the hose clamp is snug; if it’s loose, tighten it with a screwdriver.
- Make sure the hose runs without a sharp bend or pinch that could block flow. Straighten any kinks so water drains freely.
Step 3 — Confirm the knockout plug and remove it if present
- Locate the dishwasher inlet on the disposal (a small round port on the side). Use a flashlight to see if a round metal disc (the knockout) is still in place.
- If you see the plug: only remove it if the unit is powered off and you’re comfortable doing so. The usual safe approach is to turn off power, tap the plug inward with a screwdriver so it drops into the grinding chamber, then remove it from the chamber with pliers or by taking the disposal down per the manufacturer instructions.
- If you are not comfortable removing the plug or the disposal is hard-mounted, stop and call a professional to avoid damage or injury.
- After removal, reconnect the hose, tighten the clamp, and check again for kinks.
Step 4 — Clear and test
- Run hot water in the sink and run the dishwasher drain or a short disposal cycle (with power restored) to check flow. The sink should not fill while the dishwasher drains.
- If backup continues, check the P-trap and drain for clogs and clear them if accessible. If clearing the trap doesn’t help, the issue may be further in the drain line or a disposal fault.
What Not to Do
- Do not forget the disposal inlet knockout step after replacement; if unsure, a plumber is appropriate.
- Do not stick your hand into the disposal or attempt removal without cutting power first.
- Do not overtighten or clamp the hose in a way that pinches it closed; that can cause new problems.
- Do not run the dishwasher repeatedly while the sink is full — that can push water into cabinets and cause damage.
When to Call a Professional
Call a plumber if you can’t safely remove the knockout plug, if the disposal won’t reconnect correctly, if there are persistent backups after you’ve checked hose and trap, or if you see electrical or leak issues. A plumber can handle tricky removals, trapped plugs, and deeper drainline clogs.
Safety Notes
- Always turn off the disposal’s power at the breaker or unplug it before inspecting or working on the unit.
- Wear eye protection and work gloves when handling metal parts or clearing debris.
- Never operate the disposal with hands or tools inside the grinding chamber.
- If you must remove the disposal to access the knockout, follow the manufacturer’s removal instructions or hire a pro.
Common Homeowner Questions
- What if I remove the plug and it still backs up? — Check the P-trap and drain line for clogs; if clear, call a plumber.
- Can I just cut the hose shorter to fix a kink? — Only trim if you have enough hose to maintain a secure, smooth run; otherwise replace the hose to avoid leaks.
- Is it safe to tap the knockout out myself? — It can be safe if power is off and you’re comfortable; if not, have a plumber do it.