Dishwasher Not Draining? 7 Causes & How to Fix It

A dishwasher that won't drain is usually caused by a clogged filter, blocked drain hose, faulty drain pump, or garbage disposal connection issue. Most cases can be fixed in under 15 minutes without tools. Here are the 7 most common causes and step-by-step instructions to fix each one.

Why it happens: The filter at the bottom of the dishwasher traps food particles to prevent them from recirculating onto your dishes. When the filter clogs with debris, water can't drain through it. This is the #1 cause of standing water — and the easiest to fix.

Why it happens: The drain hose connects your dishwasher to the garbage disposal or sink drain pipe. Food particles, grease, and debris can accumulate inside the hose over time, creating a blockage. A kinked hose also restricts water flow.

Why it happens: The drain pump actively pushes water out of the dishwasher through the drain hose. If the pump motor fails, gets jammed by debris, or has an electrical fault, water simply stays in the tub.

How to diagnose: During the drain cycle (near the end of the wash), listen carefully. A healthy pump makes a humming or whirring sound. Complete silence means the pump isn't running. A loud grinding or buzzing suggests the impeller is jammed.

DIY check: Access the pump beneath the filter and check for obstructions — broken glass, food debris, or small objects caught in the impeller. Carefully remove any debris you find.

Why it happens: If your dishwasher drains into a garbage disposal, the installer must remove a small plastic knockout plug from the disposal's dishwasher inlet port. If this plug wasn't removed during installation, water has nowhere to go. This is the most common cause on newly installed dishwashers.

Also check: Even on older installations, the disposal itself can be clogged. Run the disposal for 15 seconds with hot water to clear any buildup before testing the dishwasher again.

Why it happens: Even if the hose isn't clogged internally, it can be kinked where it bends behind the dishwasher, or crushed where it passes through the cabinet wall. Improper installation — like routing the hose without a high loop — also causes drainage failures and backflow.

Why it happens: Many dishwashers have a check valve (also called a flapper valve) that prevents dirty water from flowing back into the tub after draining. If the valve is stuck closed, cracked, or clogged with debris, water can't exit — or drains only partially.

How to suspect this: You've cleaned the filter, checked the hose, the pump runs normally, but water still drains very slowly or only partially empties.

Why it happens: The electronic control board sends the signal to the drain pump to turn on at the right time in the cycle. If the board has a faulty relay, burned circuit, or software glitch, it may never send that signal — even though the pump itself is fine.

How to suspect this: You've ruled out all other causes. The pump works when tested directly but doesn't activate during a normal cycle. Other cycle functions may also be erratic — skipping rinse, stopping mid-cycle, or displaying error codes.

SF Bay Area: Why Dishwashers Drain Poorly in Older Homes

In San Francisco's older Victorian and Edwardian homes (built pre-1950), dishwasher drain issues are often caused by infrastructure, not the dishwasher itself:

• Cast iron drain lines: original cast iron pipes corrode from the inside, narrowing diameter over decades. A dishwasher that suddenly won't drain after years of perfect operation often has a partially blocked drain line, not a failed pump.
• Air gap placement: SF building code requires an air gap on the countertop. A clogged air gap mimics a failed drain pump exactly — water backs up into the tub. Clean the air gap cap first before calling for service.
• Garbage disposal connection: if the dishwasher drain connects to a disposal and the disposal's knockout plug was never removed during installation (a common error), the drain will back up immediately. Check this before replacing the pump.

Our technician checks all three during the $80 diagnostic visit.