If water backs up into your kitchen sink when the dishwasher runs, it’s a sign that wastewater isn’t draining properly. This problem can be alarming, especially if dirty water rises into the sink during or after a dishwasher cycle.
In most cases, this issue is caused by a drainage restriction — not a problem with the dishwasher itself.
Quick Answer:
A sink backs up when the dishwasher runs because both drain into the same pipe, and a partial clog or restricted drain can’t handle the combined water flow. As the dishwasher pumps out water, it forces water back into the sink instead of down the drain.
Why This Happens
Dishwashers typically drain into the kitchen sink’s plumbing system. When the drain line can’t keep up with the discharge, water takes the path of least resistance — back into the sink.
Common causes include:
- Partial clogs in the kitchen drain line
- Grease or food buildup in pipes
- A blocked or dirty sink P-trap
- A clogged or improperly installed air gap
- Drain hose issues
This issue often overlaps with a sink that drains slowly, even if the sink appears to work normally otherwise.
Step-by-Step: How to Address Sink Backups During Dishwasher Use
Step 1: Stop the Dishwasher if Water Backs Up
If water rises in the sink during a dishwasher cycle:
- Pause or stop the dishwasher
- Avoid running additional water
- Allow the sink to drain
Continuing the cycle can worsen the backup.
Step 2: Check the Sink Drain Speed
Run water in the sink and observe drainage.
If the sink:
- Drains slowly
- Gurgles
- Holds water
…the drain is likely partially clogged.
Step 3: Clean the Sink P-Trap
Grease and food buildup commonly collect in the kitchen P-trap.
- Place a bucket underneath
- Remove the trap carefully
- Clean out debris
- Reinstall and test
This step often resolves dishwasher-related backups.
Step 4: Inspect the Dishwasher Air Gap
If your sink has a small air gap fitting near the faucet:
- Remove the cap
- Check for debris
- Clean thoroughly
A clogged air gap can cause water to back up into the sink during dishwasher drainage.
Step 5: Avoid Chemical Drain Cleaners
Chemical cleaners can:
- Damage pipes
- Harden grease buildup
- Make future clearing harder
Mechanical cleaning is safer and more effective for kitchen drains.
What Not to Do
- Don’t ignore recurring backups
- Don’t keep running the dishwasher during backups
- Don’t pour grease down the sink
- Don’t rely on chemical drain cleaners
Backups usually worsen if left unaddressed.
When to Call a Professional
Call a plumber if:
- Backups continue after cleaning the trap
- Multiple kitchen drains are affected
- Water backs up into other fixtures
- You hear gurgling throughout the plumbing
These signs often indicate a deeper drain line blockage.
Safety Notes
- Avoid contact with dirty backup water
- Shut off the dishwasher if water rises
- Clean spills promptly to prevent damage
- Wear gloves when handling drain components
Common Homeowner Questions
Is this a dishwasher problem or a sink problem?
It’s almost always a drain issue shared by both, not a dishwasher malfunction.
Can a clogged air gap cause this?
Yes. A blocked air gap can force water back into the sink.
Will plunging the sink help?
Sometimes. If plunging helps temporarily, a partial clog is likely still present.