Dishwasher Water Backing Up Into Sink: Causes and Fixes

Dishwasher water backing up into sink almost always means the dishwasher's drain path is blocked after it leaves the machine. The usual suspects are a clogged garbage disposal, an unremoved knockout plug in a new disposal, a blocked air gap, a kinked or unlooped drain hose, or a clog in the kitchen drain. Switch off the dishwasher and check these in order.

Dishwasher water backing up into sink: how the drain works

A dishwasher pumps used water through a drain hose to the sink drain, usually via the garbage disposal or a tailpiece under the sink, and sometimes through an air gap fitted on the sink or countertop. If any part of that route is blocked, the water has nowhere to go and rises in the sink.

Quick facts: dishwasher water backing up into sink
Time needed	20 to 45 minutes
Tools	Flashlight, screwdriver, pliers, bucket, towels
Difficulty	Easy to moderate
Safety	Switch off the dishwasher and the disposal at the breaker before reaching in

Check these causes in order

Cause	Why it backs up	Fix
Clogged garbage disposal	The drain inlet is packed with food	Run and clear the disposal
Knockout plug left in a new disposal	The dishwasher inlet is sealed	Remove the plug
Blocked air gap	Debris clogs the small fitting	Clean the air gap
Kinked or unlooped drain hose	Water flows back into the dishwasher or sink	Straighten and form a high loop
Clogged sink trap or branch drain	The kitchen drain is partly blocked	Clean the trap and snake the drain

Step by step

1. Stop the dishwasher and bail out standing water from the sink and the machine floor.
2. Run the garbage disposal with cold water to clear it, after checking nothing is jammed.
3. Check for a knockout plug. If you recently fitted a new disposal, the inlet for the dishwasher hose has a plastic plug inside that must be knocked out. Switch off the power, then tap it out with a screwdriver and fish it out.
4. Clean the air gap if you have one. Lift the cap and clear the tiny passages with a brush or pin.
5. Inspect the drain hose under the sink. Make sure it is not kinked or crushed and that it rises into a high loop before connecting to the disposal or tailpiece.
6. Clean the P-trap and snake the kitchen drain if the sink drains slowly on its own.

What a high loop does

The drain hose should rise to the underside of the countertop before dropping down to the disposal inlet. This high loop prevents dirty sink water from flowing back into the dishwasher. If you have dishwasher water backing up into sink after a recent install, a missing high loop or a hose forced against the wall is a common cause.

Signs it is the disposal rather than the dishwasher

If the water in the sink is grey and gritty with food scraps, the disposal inlet is usually clogged. If it is clean-looking soapy water that returns after you run the disposal, check the air gap and hose. If both sink basins fill together, the problem is past the trap. Noting what the water looks like helps you decide which step to try first.

What not to do

• Do not use chemical drain cleaner, which can damage the dishwasher seals and leave hazardous liquid in the sink.
• Do not reach into the disposal with power on.
• Do not keep running cycles while the drain is blocked.

Prevention

Scrape plates before loading, run the disposal before starting the dishwasher, clean the air gap every few months, and flush the kitchen drain with hot water now and then. Avoid pouring grease into the sink.

When to call a plumber

Call if the backup continues after the disposal, plug, air gap and hose are checked, if the kitchen drain is slow, or if other fixtures are involved. The clog may be in the branch drain beyond your reach.

Related reading

• Dishwasher Not Draining? 8 Checks Before You Call a Repair Pro
• Garbage Disposal Not Working? Reset, Unjam and Test It
• How to Unclog a Kitchen Sink: Plunger, Trap and Snake
• Why Is My Dishwasher Leaking? 9 Sources and How to Fix Them

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