Sink Fills Up When Running Dishwasher? Do This


When a sink is filling with water when running the dishwasher, it is a sign of a clogged drain line, blocked sink air gap or an obstructed dishwasher drain hose. If you have a new garbage disposal, it is possible that the drain plug was not removed.

This is a brief guide to this post:


  • Clogged Drain Line: A clogged or partially blocked drain line can prevent water from properly draining from the dishwasher, causing it to back up into the sink.
  • Blocked Garbage Disposal: If your dishwasher is connected to a garbage disposal, a clogged disposal unit can obstruct the flow of water from the dishwasher, leading to sink backup.
  • Air Gap Issues: Problems with the dishwasher’s air gap, if it’s installed, can cause improper drainage and result in water backing up into the sink.


  • Clear Drain Line: Check for clogs or blockages in the dishwasher’s drain line. Remove any debris or buildup to allow water to flow freely. You may need to disconnect the drain hose to access and clean it.
  • Check Garbage Disposal: If your dishwasher is connected to a garbage disposal, ensure that the disposal unit is not clogged. Run the disposal to clear any blockages, and make sure the drain hose from the dishwasher is properly connected.
  • Inspect Air Gap (If Installed): If you have an air gap installed on the countertop or sink, inspect it for blockages or obstructions. Clean the air gap and hoses to ensure proper drainage.
  • Run a Short Cycle: After addressing any potential issues, run a short dishwasher cycle to check if the sink still fills up. Monitor the water flow and drainage to confirm that the problem has been resolved.
  • Professional Assistance: If the issue persists or if you’re uncertain about the cause, consider consulting a professional plumber or appliance repair technician. They can diagnose and fix any underlying plumbing problems.

1. Clogged Drain Line

As I have already mentioned, the sink and dishwasher share the same drain line. This means that if there is a problem with the drain line, it will affect both fixtures.

If the drain line is clogged, there is no way for water to pass through it and is hence forced to back up through the sink drain. It does not even have to be a full clog. A partial clog can result in the sink filling up when running the dishwasher and then the water will drain out slowly.

Usually, whenever you have a clogged kitchen sink drain, the clog is inside the P-trap. A P-trap is the U-shaped bend under the sink. All fixtures in your house have one.

Its main function is to trap potential drain clogs, preventing them from clogging the drain line farther away, where unclogging would be way harder.

Due to its shape, the P-trap is also full of water at all times. This water acts as a barrier, preventing sewer odors from coming into the house via the sink.

There are several ways to unclog a kitchen sink drain. You can consider snaking it or breaking down the clog using baking soda and vinegar.

My favorite method however involves disconnecting and cleaning the P-trap. It is an easy and fast method to have your line draining well again.

If you have a double kitchen sink, it is also the best method since it allows you to clean the tee which connects the second sink to the main drain line.

Here is how to unclog a Kitchen sink drain:

  • Start by removing the items you have stored under the kitchen sink.
  • Place a small container under the P-trap. This will prevent any water inside the trap from spilling on the floor.
  • Disconnect the P-trap. Since the P-trap has a short and long arm, I always recommend disconnecting the shorter arm to allow the water in the trap to flow out via gravity. Remember that the connections are in most cases hand-tight and you may not even need a wrench to loosen them.
  • Pull down the P-trap and inspect inside. Can you see gunk? Check also the pipe connected to the wall as well as the tail piece (pipe connected to sink drain opening). Remove as much gunk as you can.
  • Connect back the P-trap.
  • Run the dishwasher and check if wastewater has stopped backing up through the sink drain.

And basically that is how to fix a sink filling up when running a dishwasher as a result of a clogged drain line.

2. Blocked Air Gap

If water is coming out through your sink’s air gap when running a dishwasher, the air gap opening is blocked. This is usually caused by food scraps from the dishwasher as well as the use of too much or the wrong type of detergent.

In the old way of plumbing dishwashers, the drain hose used to be looped underneath the sink countertop to prevent backflow of water into the dishwasher in case of a clogged drain line.

In modern plumbing, we use a simple and effective device called a dish washer air gap. This is the knob you see next to the kitchen sink faucet.

An air gap prevents the back flow and therefore contamination of the dishwasher by wastewater in case of a clogged drain line. It works on the simple principle that water cannot siphon back through an unpressurized gap of air.

If there is a clog somewhere on the air gap, it means that the wastewater cannot flow out into the drain line and can also not flow back to the dishwasher. In that case, water will start to shoot from the top of the air gap and enter the sink.

Clearing a clog from the air gap is quite easy and fast as well. You can do it from the top or disconnect the drain house from underneath the sink and snake it from here.

3. Plugged Garbage Disposal

Have you installed a new garbage disposal lately? If you have, there is chance that you forgot to remove the drain plug and as result the dishwasher cannot drain, forcing it to fill the kitchen sink with wastewater.

New garbage disposals come with drain plugs installed on the opening where you are supposed to connect the dishwasher drain hose. They are also installed since some people need a garbage disposal but do not have a dishwasher.

If the opening is left open, the garbage disposal will be leaking every time it is being used. You should however remove the plug when hooking up the dishwasher drain.

Just grab a pair of pliers and disconnected the dishwasher drain hose from the garbage disposal. Check if the drain plug is there and pull it out. Connect back the drain hose.

And that is how to fix a dishwasher backed up into the sink. I hope this guide was helpful. If you are unable to fix it on your own, fell free to consult a professional plumber.

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