Quick Answer:
Grimy dishwater flowing back into the sink during a wash cycle usually means the dishwasher is forcing water into a drain that’s partially blocked or not venting. First stop the dishwasher, then check the sink trap and trap arm for a partial blockage that lets water back up during pump-out. If you can’t clear the trap safely or the problem returns, call a plumber.
Why This Happens
- During the dishwasher’s drain step a pump pushes water into the sink’s drain line. If the trap, trap arm, garbage disposal, or shared drain has a partial clog, that water can be forced back into the sink.
- Poor venting or a low/routed drain hose can let dishwasher flow find the easiest exit — sometimes back through the sink instead of toward the sewer.
- Problems like this are closely related to other dishwasher-sink issues, including a Sink overflows during dishwasher cycle or a clogged disposal that hides a partial trap blockage.
Step-by-Step What to Do
Step 1 — Stop the appliance and contain the mess
- Turn off the dishwasher and stop the cycle. If it won’t stop, switch off its breaker.
- Remove standing water from the sink with towels or a small cup so you can see the drain area.
Step 2 — Check the sink trap and trap arm
- Place a bucket under the trap (the curved pipe beneath the sink) and loosen the slip nuts. Remove the trap and inspect for food, grease or other debris.
- Also check the trap arm (the horizontal section that goes into the wall). A partial blockage here often allows dishwasher pump-out to push water back into the sink.
- Clear debris, rinse parts, and reassemble snugly. Make sure to retighten slip nuts by hand and test for leaks.
Step 3 — Inspect the garbage disposal and drain connections
- If you have a disposal, check that it’s not clogged and that the dishwasher drain knockout was removed when the disposal was installed.
- Run water through the disposal (power off if you’re working on it) and look for slow drainage that points to a deeper clog.
Step 4 — Check the dishwasher drain hose and air gap
- Ensure the drain hose has a high loop or is connected to an air gap. A low or kinked hose can let water run backward into the sink.
- Inspect the air gap (if present) for debris; remove the cover and clean any grit or build-up.
Step 5 — Test carefully
- Run a short drain-only cycle or a manual rinse while watching the sink. If the sink stays clear, the repair likely worked.
- If it still backs up only during the dishwasher’s pump-out, the issue may be in the shared drain line or the sewer connection.
Step 6 — Monitor and repeat if needed
- If clearing the trap fixed it, monitor the sink for a few cycles. Recurrent backups suggest a deeper blockage or venting problem that needs professional help.
- If the problem appears related to timing (only when the dishwasher finishes), consider that a separate issue like a clogged line or check valve—see notes about Backup only when dishwasher finishes.
What Not to Do
- Do not plunge the sink while the dishwasher is actively draining—you can force water where it shouldn’t go.
- Do not pour strong chemical drain cleaners into the dishwasher or disposal; they can damage components and harm you if they splash back.
- Do not remove disposal covers or reach into the disposal when it is powered. Always disconnect power first.
When to Call a Professional
- Repeated backups after you clear the trap or if multiple fixtures are slow or backing up — possible mainline or sewer issue.
- Foul sewage smell, visible sewage in drains, or water backing up into other drains — call immediately.
- If you can’t access or safely clear the trap and trap arm, or if the dishwasher drain hose or venting appears to require re-routing.
Safety Notes
- Turn off the dishwasher and, if you’ll work on the disposal, switch off its power at the breaker before starting work.
- Wear gloves and eye protection when handling drain parts or trapped food and grease.
- Avoid chemical drain cleaners with dishwashers and disposals; they can splash and burn skin or damage seals.
Common Homeowner Questions
- Why does this happen only during the dishwasher cycle?
A: The dishwasher pump forces a lot of water into the drain at once; a partial clog or poor venting lets that water back up into the sink. - Can I run the dishwasher again after clearing the trap?
A: Yes — run a short drain cycle to test. Stop immediately if you see more backflow. - Will a plumber need to open my wall to fix this?
A: Not usually. Many problems are in the trap, trap arm, disposal, or drain hose. A plumber will inspect and advise if any wall access is needed.