Quick Answer:
Stop the dishwasher cycle and contain the water immediately. Once you have the water under control, run plain tap water in the sink to compare the drain flow. If the sink drains slowly even with only faucet water, you likely have a clog in the sink drain or disposal. If the sink only overflows when the dishwasher pumps, the dishwasher drain hose, check valve, or air gap is the usual culprit.
Why This Happens
Two things must be true for a sink to overflow during the dishwasher cycle: the dishwasher is trying to pump waste water into the house drain, and that drain is partially or fully blocked so the water has no place to go. Common causes:
- Food, grease or debris blocking the sink drain or garbage disposal.
- Clog or slow section further down the drain line (trap, branch, or main line).
- Dishwasher drain hose kinked, clogged, or installed without a proper high loop or air gap so water can backflow into the sink.
- A failed check valve or non-return piece in the dishwasher drain path.
If the overflow happens only when the dishwasher pumps, it’s often not a general house plumbing failure but a backup related to the dishwasher’s drain route — think “Backup only when dishwasher drains.”
Step-by-Step What to Do
1. Stop the cycle and contain the water
Immediately cancel or pause the dishwasher and open the dishwasher door just enough to stop the pump. If the machine won’t stop, turn it off at the control or unplug it. Put towels and a bucket under the sink, and use absorbent towels to keep water from running into the cabinet. This protects the cabinet and flooring from damage.
2. Turn off water and power if needed
If the overflow is large or water is near electrical outlets, shut off the dishwasher’s power at the switch or breaker. If water is coming from a hot-water connection or a visible supply line, consider shutting off the sink’s shutoff valves until the situation is controlled.
3. Contain and dry the area
- Wipe up or soak up water with towels and place a bucket under drips.
- Remove wet items from the cabinet and set them aside to dry.
- Open cabinet doors for ventilation and use a fan or dehumidifier if you have one.
4. Test sink drainage with plain faucet water
Run the sink faucet on a steady moderate flow and watch how quickly the water drains. This simple test helps you compare flow rates: if the sink drains normally with only faucet water, the problem is likely tied to the dishwasher’s drain hose, air gap, or check valve. If the sink drains slowly or backs up with only faucet water, you have a general sink drain clog to address.
5. Clear obvious blockages safely
With the flow test done, remove visible debris from the sink strainer and, if you have a garbage disposal, make sure it’s off and clear large pieces of trapped food. Do not stick your hand into the disposal. If comfortable, reset the disposal with its reset button after power is off and the area is dry.
6. Try another controlled dishwasher run
Only after the sink drains normally with faucet water and the area is dry, run a short rinse-only dishwasher cycle while watching the sink. If overflow returns, stop immediately and proceed to the next steps or call a pro.
What Not to Do
- Do not let repeated overflows soak the cabinet; shut things down and dry the area immediately.
- Don’t keep running the dishwasher hoping the problem will clear itself—this risks water damage and mold.
- Don’t reach into a garbage disposal or work on electrical parts while the area is wet or the appliance is powered.
- Avoid dumping strong chemical drain cleaners down a sink that’s connected to a dishwasher; they can damage components and create hazardous conditions.
When to Call a Professional
Call a plumber if:
- The sink still drains slowly with faucet water or backs up into other fixtures.
- You get repeated overflows or you find water damage inside the cabinet.
- You’ve checked visible parts (strainer, disposal, air gap) and the dishwasher still forces water into the sink during pump-out.
- You notice sewage smell, persistent leaks, or standing water in the drain lines. Also consider help if this happens intermittently at night — for example, if the sink keeps filling when appliances run unattended: Sink fills when dishwasher drains overnight.
Safety Notes
- Avoid electrical contact with standing water. Shut off power if water reaches outlets or appliance cords.
- Use gloves when handling wet insulation, moldy materials, or dirty water.
- If water has soaked cabinet interiors, remove wet insulation or porous materials and dry them quickly to reduce mold risk.
- Do not attempt complex electrical or gas appliance repairs yourself. Leave those to licensed technicians.
Common Homeowner Questions
- Why does this only happen during the dishwasher cycle?
Because the dishwasher pumps water into the shared drain. If that path is blocked or the dishwasher’s drain plumbing lacks a proper high loop/air gap, water will back up into the sink. - Can I run the dishwasher again right away?
No. Stop the machine, dry the area, and test the sink drain with plain water first. Running it again risks more overflow and damage. - Are chemical drain cleaners a good fix?
Not recommended. They can harm your dishwasher, disposal, and pipes, and are hazardous to handle. Call a plumber for persistent clogs.