A kitchen sink usually backs up when the dishwasher runs because both share the same drain, and that drain is partly clogged. When the dishwasher pumps out its water, there is nowhere for it to go except up into the sink.
How the Two Are Connected
Most dishwashers drain through a hose that connects to the garbage disposal or to the sink drain pipe under the cabinet. Some homes also have an air gap, a small fitting on the countertop that keeps dirty water from flowing back into the dishwasher. If any part of this path is blocked, water ends up in the sink.
Typical Culprits Under the Sink
- Grease and food buildup: the most common cause in kitchen drains.
- A clogged disposal: food packed in the disposal blocks the dishwasher inlet.
- A new disposal: if a disposal was just installed, a small knockout plug inside may not have been removed.
- A blocked air gap: food can clog the air gap, causing water to spill out of it or into the sink.
- A kinked drain hose: the hose under the sink can bend and slow the flow.
Quick Checks Under the Sink
Run the disposal with cold water to clear any packed food. Clean the air gap cap if you have one. Check under the sink for a kinked or sagging hose. Avoid chemical drain cleaners, since they can damage the disposal and the dishwasher.
When It Is a Bigger Problem
If the sink stays slow even when the dishwasher is off, or other drains back up too, the clog is likely further down the line. Grease buildup in the pipe behind the wall usually needs professional cleaning.
Scraping plates into the trash and keeping grease out of the sink helps prevent the problem from returning.
If the backups keep happening, clearing grease from the kitchen line is the long-term fix. For dishwasher hookup questions, you can have a plumber check the connections at (833) 603-1840.