Quick Answer:
If the sink fills or backs up right after you installed a garbage disposal, stop using the unit and check a few common installation issues: the dishwasher knockout plug, the position of the trap, and trapped debris in the trap or tailpiece. Don’t run the disposal continuously against a backed-up sink—turn it off and follow the steps below to find the problem.
Why This Happens
- New installations can leave a metal knockout plug in the dishwasher inlet; if a dishwasher is connected and that plug isn’t removed, water has nowhere to go and the sink can back up.
- The drain trap must sit on the disposal outlet. If the trap is installed upstream (on the sink tailpiece before the disposal), the disposal will push water against the trap and the sink will back up.
- During construction or installation, debris (plumbing tape, packaging, small metal shavings) can lodge in the trap or tailpiece and block flow.
- Less commonly, the clog is farther downstream than the trap—then running the disposal won’t clear it and you’ll need professional help.
Step-by-Step What to Do
1. Stop and power down
- Turn the disposal off. If it’s fed from a wall switch, switch it off; for extra safety, shut off the breaker or unplug the unit before touching plumbing components.
2. Check the dishwasher knockout (if a dishwasher is hooked up)
- Locate the small round inlet on the side of the disposal where the dishwasher hose connects. If the dishwasher is connected and the inlet still has a solid metal plug, it must be removed per the disposal instructions—otherwise the dishwasher and sink will back up.
3. Verify the trap location
- Look under the sink to confirm the trap is mounted directly to the disposal outlet. The trap should come off the disposal, not off the sink tailpiece before the disposal.
- If the trap is upstream of the disposal, reposition the piping so the trap attaches to the disposal outlet.
4. Remove and inspect the trap
- Place a bucket under the trap to catch water (this is the step that tests flow visually).
- Loosen the slip nuts on the trap and remove it. Inspect for construction debris, packaging, or anything that would block flow.
- Clean the trap thoroughly and clear any debris found. Replace the trap, hand-tighten slip nuts, and test for leaks.
5. Test flow with the tailpiece bucket check
- With the bucket still under the tailpiece or trap area, run water briefly from the sink. If water flows out into the bucket freely, the immediate outlet is clear.
- If you removed the trap and water still doesn’t flow from the disposal outlet, the blockage is likely upstream or inside the disposal. If water flows from the disposal outlet but the sink still backs up when the dishwasher runs, recheck the knockout plug.
6. Reconnect and test the disposal
- Reattach the trap and tighten connections. Restore power to the disposal and run a small amount of water while using the disposal briefly to confirm proper draining. If the sink backs up immediately, stop and proceed to the next checks or call a professional.
What Not to Do
- Don’t keep running the disposal against a backing sink—overheating can damage the unit.
- Don’t stick your hand into the disposal. Always disconnect power before reaching into or removing parts.
- Don’t pour strong chemical drain cleaners into a new disposal—these can damage components and create hazardous conditions. If the clog appears to be downstream beyond the trap, a plumber is appropriate.
When to Call a Professional
- Call a plumber if you find the clog is past the trap or in the house drain line. Professionals have the tools to clear deep blockages without risking damage.
- If you suspect the disposal was miswired electrically or is making unusual noises after these checks, contact an electrician or plumber with appliance experience.
- If you’re uncomfortable working under the sink or disconnecting plumbing, hire a plumber to avoid leaks or improper reassembly.
Safety Notes
- Always cut power to the disposal before removing the trap or working on the unit—use the wall switch and, if possible, turn off the breaker.
- Wear safety gloves and eye protection when handling debris and when removing trapped items.
- Keep tools and loose parts away from the disposal opening; don’t use the disposal to grind anything while you’re troubleshooting.
If you notice unusual smells or sounds after working on the unit, check the seals and connections and consider reading about Odor after disposal replacement or Gurgling after disposal install for related troubleshooting ideas.
Common Homeowner Questions
- Why does the dishwasher back up into the sink? Often the dishwasher inlet knockout was left in place or the hose is kinked; remove the knockout and make sure the hose slopes toward the disposal inlet.
- Can I clear this clog with a plunger? A plunger can help if the clog is in the trap or sink area, but don’t use a plunger while the disposal is powered or if you suspect the clog is deep in the drain line.
- How do I know if the disposal outlet is misconnected? Inspect under the sink: the trap must connect to the disposal outlet. If plumbing routes water around the disposal or the trap is upstream, that’s a misconnection and should be corrected.