It shows up two ways. Either the dishwasher drains and gray water rises in the sink basin, or you run water in the sink and it ends up sitting in the bottom of the dishwasher. Both are among the most common kitchen questions homeowners ask online, and both usually come down to the same thing: the dishwasher and the sink share one drain, and something in it has narrowed.
How a dishwasher actually drains
A dishwasher doesn't have its own line to the sewer. Its drain hose connects to the kitchen sink plumbing — either into the garbage disposal or into a fitting on the sink's tailpiece — and from there both share the trap and the drain line in the wall. When the dishwasher pump pushes water out faster than that shared line can carry it away, the water takes the easiest path, which is up into the sink.
The common causes
- A partial clog in the shared drain. Grease, food particles and soap build up in the trap and the line behind it. The sink may drain slowly but acceptably on its own and only overflow when the dishwasher adds a surge. This is the most common cause.
- A clogged garbage disposal. If the dishwasher drains into the disposal and the disposal chamber or its outlet is packed with food waste, the dishwasher water backs up there first. A new disposal installed without removing the knockout plug on the dishwasher inlet causes the same symptom.
- No high loop or air gap. The drain hose should rise up high under the counter (a "high loop") or run through an air gap fitting before it drops to the drain connection. Without that, dirty sink water can flow backward down the hose into the dishwasher.
- A clogged air gap. If you have the small chrome cap on the sink deck and water spurts out of it when the dishwasher drains, the air gap or the hose past it is blocked.
What you can check yourself
- Run the disposal with cold water for a bit, then see if the dishwasher drains normally.
- Look under the sink: the drain hose should loop up as high as it can go before it connects. If it sags to the floor of the cabinet, lift and secure it.
- If you have an air gap, pull the cap and clear any debris inside.
- Clean the dishwasher's own filter per the manufacturer's instructions — a clogged filter can mimic a drain problem.
What not to do
Avoid pouring chemical drain cleaner into a sink that's connected to a dishwasher and disposal. It can sit in the disposal, pool in the dishwasher, and it rarely clears grease that has built up further down the line. And if the sink is full of dishwasher water, don't start another cycle to "push it through."
When it's the line, not the connection
If the hose is looped correctly, the disposal is clear, and the sink still fills when the dishwasher drains, the restriction is in the kitchen drain line itself. That's grease buildup on the pipe wall in most homes, and it tends to return after a quick fix. We cover the pattern in why a kitchen drain keeps clogging. If other fixtures in the house are slow at the same time, the problem may be further down — see signs you need main line drain cleaning.
Getting it cleared properly
A kitchen line clogged with grease is best cleared all the way through, not just opened up at the trap. For heavy buildup, jetting instead of snaking gives a longer-lasting result. See kitchen drain cleaning in Bend or call (888) 616-9423.