Quick Answer:
If the sink basin slowly rises whenever the dishwasher runs, the dishwasher and sink are sharing a drain and the drain is partially blocked or cannot handle the dishwasher’s flow. Stop the dishwasher if the sink is rising, then run a cycle while watching the sink water level and the cycle timing to see exactly when water backs up.
Why This Happens
Most kitchens share a drain line for the sink and dishwasher. If that shared path is partly clogged, the dishwasher can push water back into the sink during its drain stage or while it refills. Other common contributors are a blocked garbage disposal, a kinked or poorly installed dishwasher drain hose, a clogged sink tailpiece or P-trap, or a venting problem that slows drainage. For related symptoms, homeowners often search for Dirty water backing into sink or notice noises described in Dishwasher causes kitchen sink gurgling.
Step-by-Step What to Do
Step 1 — Prepare and watch a cycle
- Clear the sink of dishes and debris so you can see the water level.
- Run the dishwasher and watch both the sink water level and the timing (fill vs wash vs drain) to pinpoint when it rises. Note whether it rises during the dishwasher’s drain phase, when it refills, or at another point.
- If the sink starts rising, stop the dishwasher immediately to prevent overflow.
Step 2 — Check visible points of blockage
- Look into the sink strainer and the garbage disposal inlet for food or debris you can remove by hand.
- If you have a garbage disposal, run it briefly (only when water level is low) to clear trapped food—follow the disposal manufacturer’s safety steps first.
Step 3 — Test simple fixes
- Use a plunger on the sink bowl to try to dislodge a clog in the sink side. Seal the other basin or drain opening if you have a double sink.
- After plunging, run a small amount of water and check if the dishwasher drain cycle still backs up.
Step 4 — Inspect the P-trap and hoses
- Place a bucket under the P-trap, then loosen the trap nuts and remove it to check for blockages. Clean and reassemble the trap if you are comfortable doing so.
- Check the dishwasher drain hose for kinks, clogs, or an improperly installed high loop/air gap. If the hose is clogged, you may be able to clear it with a flexible brush or by flushing from the sink end.
Step 5 — Re-test and observe
- Reassemble anything you opened, then run a short dishwasher cycle while watching the sink. Note whether the problem is resolved or if backup still occurs at the same point in the cycle.
What Not to Do
- Do not keep running cycles while the sink is rising; stop and address drainage first.
- Do not pour large amounts of chemical drain cleaner into a sink connected to the dishwasher—these chemicals can damage hoses and seals and are hazardous if they back up into appliances.
- Do not remove or work on electrical parts of the dishwasher or garbage disposal while they are plugged in or powered.
When to Call a Professional
- Call a plumber if you cannot clear the clog with plunging or removing the P-trap, or if the same backup happens repeatedly.
- Call a professional if multiple drains are slow or if you smell sewage—those signs point to a larger drain or sewer issue.
- If the dishwasher drain hose or disposal is damaged or you’re unsure how to safely disconnect and reattach plumbing, get professional help.
Safety Notes
- Turn off the dishwasher and any power to the garbage disposal before working under the sink.
- Wear gloves and eye protection when handling debris or opening traps.
- Avoid mixing different chemical drain cleaners. If you’ve used a chemical cleaner, wait and ventilate the area before attempting manual cleaning, and consider calling a pro instead.
Common Homeowner Questions
- Q: Why does the sink only fill when the dishwasher runs?
A: Because the dishwasher’s drain discharge is overwhelming a partial clog or restriction in the shared drain at a specific point in its cycle. - Q: Can I keep using the dishwasher until a plumber arrives?
A: No—stop running it when you see backup to avoid overflow and further damage. - Q: Will a plunger fix it?
A: A plunger can often clear simple clogs in the sink drain and may stop the backup; if it doesn’t, more inspection or a plumber is needed.