Quick Answer:
Most often the sink bowl backs up during dishwasher drain because of a partial clog or a poor drain connection that lets dishwasher discharge flow into the sink. Run the dishwasher by itself and watch the sink during the drain-out to confirm whether the water is backing up only at drain time.
Why This Happens
Dishwashers share the sink drain or disposal. When the dishwasher pumps out water it must go through the same trap and drain line the sink uses. If that path is partially blocked, or the drain hose is routed incorrectly, the dishwasher’s outgoing water will push into the sink bowl instead of flowing away. Other triggers include a clogged sink trap, a blocked branch in the house drain, or a new disposal/connection that changed flow direction — check **Cross-flow after installing disposal**. Less commonly, pressure or flow changes after a water outage can expose weak points in the drain system — see **Cross-flow after city outage**.
Step-by-Step What to Do
1. Clear standing water and prepare
- Remove dishes and clear the sink so you can see any backflow.
- Place a bucket or towels nearby in case water comes up.
2. Run the dishwasher alone and observe the sink during drain-out
- Start the dishwasher on a drain or normal cycle with no other water running in the house.
- Watch the sink as the dishwasher pumps out. Note whether the sink water rises quickly only when the dishwasher is draining.
- If you see a small surge that then drains away, that suggests a minor restriction. If the sink fills and stays full, that points to a clog or blocked house drain.
3. Check the sink P-trap and disposal
- If the sink drains slowly at other times, remove and inspect the P-trap for trapped debris. Clear it, then re-test.
- If you have a garbage disposal, make sure the dishwasher hose is secured to the disposal inlet and the disposal knock-out was removed during installation.
4. Inspect the drain hose routing
- Confirm the dishwasher drain hose makes a high loop under the counter or runs to an air gap if required. A low hose can allow dishwasher water to flow into the sink.
5. Test again and try simple clearing methods
- After removing obvious blockages, run the dishwasher again and watch the sink during drain-out.
- If slow drainage persists, try using a plunger on the sink or a drain snake on the trap. Don’t force chemicals into a backed-up system if you suspect a major clog or if the dishwasher is full of water.
What Not to Do
- Do not run the dishwasher repeatedly while backups occur.
- Do not stick your hand into a disposal or reach into a drain without shutting off power and wearing protective gloves.
- Avoid pouring large amounts of drain-cleaning chemicals into a system that may be connected to the dishwasher; this can damage components or create hazardous fumes.
When to Call a Professional
- Calls are justified if the sink keeps filling during every dishwasher drain after you’ve cleared the trap and checked hoses.
- Contact a plumber if you notice sewage smell, backups in multiple fixtures, or if a snake/plunger does not clear the blockage.
- Also call a pro if you’re uncomfortable disassembling the trap or working near the disposal motor and electrical connections.
Safety Notes
- Turn off power to the disposal at the breaker before working on it. Never work on the disposal while it is plugged in or powered.
- Wear gloves and eye protection when handling drain debris or using a drain snake.
- Be cautious with hot water and avoid mixing chemical cleaners; chemical reactions can be hazardous.
Common Homeowner Questions
- Q: Will running the sink while the dishwasher drains help?
A: Sometimes running a little water into the sink can keep flow moving, but it won’t solve an underlying clog. - Q: Can the dishwasher force sewage back into the sink?
A: It can push water into the sink if the drain is blocked, but true sewage backflow is usually a sign of a larger sewer line problem and needs a plumber. - Q: Is a clogged P-trap the common cause?
A: Yes; the P-trap or the immediate branch drain is a common, easily fixed source of backups during dishwasher drainage.