Why is my dishwasher not draining?

Start with the cheapest and most likely cause and work down the list. Roughly two-thirds of "won't drain" calls trace back to a part you can reach without tools.

1. 1The filter. A clogged filter is the leading reason a dishwasher stops draining. Food, grease, and bits of paper collect in the fine mesh at the bottom of the tub and choke the flow.
2. 2The drain hose. A kink, a sag, or a missing high loop lets dirty water sit or siphon back in.
3. 3The air gap. The small chrome cap on the counter behind the faucet can fill with debris and stop the discharge.
4. 4The disposal knockout plug. On a newly installed disposal, a plastic plug seals the dishwasher inlet until someone taps it out. Leave it in and the water has no exit.
5. 5The disposal or drain line. A disposal full of food, or a clogged sink drain, backs the water up into the dishwasher.
6. 6The check valve or drain pump. A failed one-way valve lets water flow back, and a burnt-out pump cannot move water at all.

Before you touch anything, turn the dishwasher off and unplug it or switch off its breaker. Scoop out the standing water with a cup and a sponge so you can work in a dry tub.

The filter is the first thing to check because it is the most common cause and the easiest to fix. Whirlpool puts it plainly in its troubleshooting guide: "The most common cause of poor draining is a clogged filter." The part sits in a recess in the floor of the tub, usually under the bottom spray arm.

Twist the cylindrical filter a quarter turn counterclockwise and lift it out. Many models pair it with a flat lower filter that lifts out next. Rinse both under warm running water and scrub the mesh with a soft brush to clear trapped grease and food. Look in the empty filter housing for glass shards, fruit pits, or a bone that slipped past, and wipe the area clean. Seat the filter back in and lock it with a clockwise turn until it stops.

In Phoenix, hard water makes this job matter more. City of Phoenix water runs about 10 to 17 grains per gallon, which the USGS rates as hard to very hard. The dissolved calcium and magnesium leave a chalky scale on the filter mesh and tub that traps debris faster than soft water would. Soaking the filter in white vinegar for a few minutes loosens that buildup. Cleaning the filter monthly keeps it from clogging again.

If a clean filter does not solve it, follow the water's path out of the machine.

Check the drain hose for a kink or a missing high loop. The hose runs from the back of the dishwasher to the disposal or air gap. Pull the machine out far enough to see it, or look under the sink. Straighten any sharp bend. The hose should rise in a high loop up to the underside of the counter before it drops to the drain connection, or connect through an air gap. That loop stops dirty sink water from siphoning back into a clean dishwasher. A hose lying flat on the cabinet floor is a frequent culprit.

Clean the air gap. Arizona builds to the Uniform Plumbing Code, and UPC Section 807.3 states that no dishwasher "shall be directly connected to a drainage system or food waste disposer without the use of an approved dishwasher air gap fitting on the discharge side." That fitting is the slotted chrome cap next to your faucet. It is required here, not optional, and it is the part that keeps sewage from back-siphoning into your dishes. Twist off the cap and the cover underneath, then pull out the insert and rinse away the gunk. If water sprays onto the counter from the air gap during a cycle, that points to a clog downstream in the disposal or drain line, not the air gap itself.

Tap out the disposal knockout plug on a new disposal. This catches many people after a kitchen remodel. A new garbage disposal ships with the dishwasher inlet sealed by a molded plastic knockout plug, and it must be removed before the dishwasher can drain into it. InSinkErator's install instructions direct the installer to "use a screwdriver and hammer to knock out the plug" inside the dishwasher inlet, then fish the loose plug out of the disposal so it does not jam the unit. If your dishwasher never drained from the day a disposal went in, this is almost certainly why.

When the disposal itself or the sink drain is blocked, the dishwasher cannot empty into it, so the water rises back into the tub. Run the disposal with cold water and listen. If it grinds normally but the sink drains slowly, the clog is downstream in the trap or drain line rather than in the disposal.

A common and quiet cause is disposal sludge from running it without enough water. InSinkErator advises that you should "run cold water before, during, and after you use your disposal," and that the water "moves the waste all the way down the drain lines." Hot water melts grease that then re-hardens further down the pipe, so cold is the rule. Grind a sink full of ice cubes to knock loose buildup, then flush with a strong stream of cold water.

If the sink still drains slowly, the P-trap under the sink is the next suspect. Place a bucket beneath it, unscrew the slip nuts, and clear the trapped food and grease. A drain snake clears a clog farther down the line. Skip chemical drain cleaners here. They can sit in standing water against your dishwasher hose and pump seals, and they create a hazard for anyone who later opens the line.

Work through the filter, hose, air gap, knockout plug, disposal, and trap first. If the tub still holds water after all six, the problem is usually inside the machine: a failed check valve that lets drained water flow back, or a worn drain pump that no longer moves water. You may hear the pump hum without pushing water, or find no clog anywhere in the visible path.

These repairs mean pulling the dishwasher, removing the lower panel, and testing or swapping a pump, which is the point where a licensed plumber or appliance tech earns the call. Phoenix's hard water also wears pumps and valves faster, since scale builds up on the moving parts. If you see water under the cabinet, a leak at the hose connections, or a backup that affects the sink and dishwasher together, treat it as a drain-line problem and have it looked at before it spreads.

HQ Plumbing & Air works throughout metro Phoenix and offers 24/7 service for drain backups that will not clear. If the dishwasher and the kitchen sink are both backing up, that points to a shared drain clog worth clearing properly.

For related fixes, see our guides on what a dishwasher air gap is, why a garbage disposal drains slowly, and a garbage disposal that hums but will not work.