Quick Answer:
If your sink overflows while the dishwasher drains, it usually means water is trying to use the same drain path but something downstream or in the connection is blocked or restricted. Stop the dishwasher, check the drain route, and run the dishwasher drain cycle while you watch the sink to confirm where water is coming from. If the overflow continues or gets worse, turn off the dishwasher and call a plumber.
Why This Happens
- Dishwashers and kitchen sinks commonly share a drain line. If that line is partially blocked, dishwasher discharge can back up into the sink basin.
- Partial clogs in the P-trap, drain pipe, or a clogged garbage disposal can force water to the path of least resistance: the sink.
- If venting is poor or a downstream sewer line is clogged, both fixtures can act oddly or overflow during high flow.
- Sometimes the symptom is similar to other issues, such as Sink backs up only when bathtub drains or Sink backs up when water heater refills, which point to shared drain or sewer problems rather than the dishwasher itself.
Step-by-Step What to Do
Confirm the problem safely
Stop the dishwasher and remove standing water from the sink so you can watch what happens next without extra water hiding the source.
Run the dishwasher drain cycle and observe the sink
Start the dishwasher’s drain-only or final cycle and watch the sink and drain connection closely. Note whether water backs up into the sink as the dishwasher drains, how fast it comes up, and whether it drains again when the dishwasher pump stops.
Check the garbage disposal and inlet hose
Make sure the garbage disposal (if present) is clear and that the dishwasher drain hose is properly connected to the disposal or tailpiece. If the disposal is jammed, clear it with the power off and the breaker off for safety.
Clear the P-trap and accessible trap area
Place a bucket under the sink, loosen the trap nuts, and remove the P-trap to clear debris. Reassemble and test the drain. If you aren’t comfortable doing this, skip to calling a pro.
Try a safe drain clearing method
Use a plunger on the kitchen sink or a hand-crank drain snake in the drain line (not the main sewer line) to remove clogs. Avoid pouring harsh chemicals because they can damage pipes and create hazards.
Test and observe again
After any cleaning, run the dishwasher drain cycle again and watch the sink. If the sink no longer overflows, run a normal dishwasher cycle to be sure the problem is resolved.
What Not to Do
- Do not continue using the dishwasher if overflow worsens. Stop it immediately to prevent water damage.
- Do not pour large amounts of chemical drain cleaner into the drain while the dishwasher or sink is overflowing. They can splash back and cause injury or damage.
- Do not reach into garbage disposal or drain openings while the power is on. Always shut off power at the breaker before working on the disposal.
When to Call a Professional
- If you can’t clear a clog with simple tools (plunger, hand snake) or the trap cleaning didn’t help.
- If water backs up into other drains or fixtures, or if the overflow is strong and won’t stop — this may indicate a sewer line or main drain problem.
- If the dishwasher hose, disposal, or plumbing connections appear broken or are leaking and you’re unsure how to repair them safely.
Safety Notes
- Turn off power to the dishwasher and garbage disposal at the breaker before working on electrical components.
- Wear gloves and eye protection when clearing traps or snaking drains. Wastewater can contain bacteria and debris.
- Avoid chemical drain cleaners when there is standing water or when storms of overflow are present — they can splash and cause burns.
- If you suspect a sewer backup (strong sewage smell, waste in drains), evacuate the affected area and call a professional immediately.
Common Homeowner Questions
- Why does the sink only overflow when the dishwasher runs?
Because the dishwasher discharges a lot of water quickly into the shared drain; a partial clog or restricted line forces that water back up into the sink. - Can I fix this myself?
Sometimes — clearing the P-trap, running a snake, or clearing the disposal can help. Stop and call a plumber if unsure or if the problem repeats. - Is this an emergency?
If water is overflowing and you can’t stop it, or if sewage is backing up, treat it as urgent and contact a plumber right away to avoid damage and health risks.
For more related articles, see the Sink Backs Up Only When Other Fixtures Run hub.