How to Troubleshoot a Dishwasher That Won't Drain

Woman unloading dishwasher in kitchen.
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A dishwasher that fails to drain correctly is tricky to diagnose because there are lots of possible reasons for the problem, and some don't even involve the dishwasher. Sometimes, it involves issues with the plumbing itself that need to be addressed before you turn to repairing or replacing the dishwasher.

Note: Be aware that it is normal for some dishwashers to retain a small amount of water at the bottom of the tub after a complete cycle. If you have just a small pool in your machine, check the owner's manual (or call the manufacturer) to see if this is normal.

Plumbing-Related Causes

Surprisingly often, drain problems with a dishwasher actually originate in drain parts outside the appliance itself.

Tools and Supplies You Will Need

  • Assorted wrenches and screwdrivers
  • Air gap fitting (if needed)

Instructions

Not all the following steps will need to be completed. Continue until you determine the cause of the dishwasher's drainage problems. If none of these plumbing-related issues seems to be at fault, then proceed to the dishwasher itself.

Check the Garbage Disposal

In most cases, the dishwasher drain hose connects to the disposal. When there is food built up inside the disposal, or when the disposal is clogged, there is less space for the dishwasher water to go. Run the disposer thoroughly with plenty of water flowing to clear it out. This may well fix the drainage problem with the dishwasher.

Clean the Air Gap

This is the small (usually chrome) cylinder on the back edge of the kitchen sink. By preventing air vacuums, it is designed to prevent drain water from siphoning back into the dishwasher. If water comes out of the air gap (and sometimes all over the counter) when you run the dishwasher, the blockage is between the air gap and the garbage disposer. To clear it, remove the air gap cap and clean inside the air gap. Next, disconnect the hose that runs from the nipple on the bottom of the air gap to the garbage disposal or sink drain and make sure that it is not clogged.

Check the High-Loop

If your dishwasher doesn't have an air gap, its drain hose may be looped up and attached to the bottom of the countertop near the sink. Although this is not code-compliant in most places, it serves the same purpose as the air gap—to prevent drain water from siphoning back into the dishwasher. If this hose has come loose so it is no longer looped up higher than the level of the dishwasher, it may prevent water from properly draining out of the appliance. Returning the hose to its proper high-loop position will fix the problem; or, you can install a proper air gap.

Note: If you've just installed a new garbage disposal, make sure the plug inside the drain hose nipple has been knocked out. New disposals have a little plastic disc covering the drain opening, which must be punched out with a hammer and screwdriver; unless you do, the dishwasher cannot drain into the disposal. The disc falls into the disposer's grinding chamber, where you can fish it out. 

Appliance-Related Causes

If there are no obvious plumbing-related causes, drainage problems may originate with several possible components on the appliance itself.

Tools and Supplies You Will Need

  • Bucket and sponge
  • Screwdrivers
  • Replacement drain hose (if needed)

Instructions

Check the Drain Screen

Check the drain screen at the bottom of the dishwasher tub and make sure nothing is clogging it. Clear plastic lids, glass, labels, and bits of plastic can cover the drain, and they can be hard to see unless you look closely.

Clean the Filter

Dishwashers use a variety of different filter designs. Some have a simple screen, some have removable filters below the screen, and some require removing the bottom spray arm for a thorough cleaning. Check the appliance manual for instructions on how yours should be cleaned.

Check the Dishwasher Drain Hose

This is usually a corrugated plastic tube that runs from the bottom of the dishwasher to the air gap, garbage disposal, or sink drain, depending on the setup. Turn off the power to the dishwasher by pulling out its plug or switching off the breaker to the dishwasher circuit. Remove the toe-kick panel, then loosen the hose clamp securing the drain hose to the dishwasher pump; have a sponge handy for any water that spills out. Clean the end of the hose, then blow into it. If the hose is plugged and you can't blow through it, remove the clamp on the other end of the hose, then clean the hose out in a sink. If you can't unclog the hose, or if it is old and cracked, replace it.

Call a Repairman

If you've tried all of the above fixes and the dishwasher is still not draining, there are several dishwasher parts that could be at fault. Checking many of these parts requires a multimeter and some knowledge of electrical circuitry. It's best to leave this to a pro unless you have the expertise with this kind of diagnosis. Most dishwasher replacement parts are widely available online, using the machine's model number to find the right parts. Likely culprits include:

  • Check valve
  • Piston and nut assembly (on drain sump)
  • Drain pump and drain motor
  • Pump solenoid
  • Belt (on belt-driven pumps)
  • Timer
  • Door switch
  • Electronic control

Replace the Dishwasher?

Appliance repair specialists can be expensive, and if your dishwasher is more than nine or 10 years old (the average life expectancy) and none of the obvious fixes remedies the drainage problem, it may be time to replace the dishwasher. This is not as hard as it sounds, and many homeowners do this work themselves. Some appliance stores will install the dishwasher free of charge as part of the purchase price and they may even handle the disposal of the old appliance.