Quick Answer:
Most often this happens because the dishwasher drains through the garbage disposal and something at the hose connection or inside the disposal is blocking or tripping the unit. First, watch the disposal while the dishwasher drains and check the dishwasher hose connection and the knockout plug at the disposal inlet. Those quick checks usually show whether it’s a clog, a plugged inlet, or an electrical/overload issue.
Why This Happens
Dishwashers usually empty into the sink drain or into the garbage disposal. When the dishwasher pumps water, that flow can reveal problems the rest of the time: a blocked inlet, a still-in-place knockout plug, a kinked hose, or a disposal that trips its overload when extra water and food reach it. If the disposal is jammed, or its reset switch trips when the dishwasher runs, the dishwasher drain can cause the disposal to stop working until the jam or overload is cleared.
Step-by-Step What to Do
1. Observe the disposal while the dishwasher drains
- Run a short drain or rinse cycle and stand at the sink while it drains. Have someone run the dishwasher if you cannot see both the dishwasher and disposal at once.
- Watch and listen under the sink: does the disposal try to run, hum, click, or go dead when the dishwasher pumps? Look for water backing up into the sink.
- If you see water backing up or a humming motor that stops, note that behavior—it’s a clue whether the problem is a clog or an electrical trip.
2. Check the dishwasher hose connection and knockout plug status
- Open the cabinet under the sink and find the dishwasher drain hose where it connects to the disposal inlet.
- Confirm the hose is clamped securely, not kinked, and routed so water can flow downhill into the disposal.
- Look at the disposal inlet where the hose attaches. The removable knockout plug should be removed when the disposal was installed to accept the dishwasher hose. If you see a plastic plug inside the inlet or no clear opening, the plug may still be in place.
- Do not try to hammer out a knockout plug while the unit is installed and powered. If the plug is present and you’re comfortable doing the safe steps below, turn off power at the breaker first before attempting removal; otherwise call a pro.
3. Check for jams, reset, and electrical issues
- Before placing your hand near the unit, switch the disposal off and cut power at the breaker. Use a flashlight to inspect the grinding chamber for obvious jams.
- Try the disposal’s reset button (usually on the bottom). If it trips repeatedly when the dishwasher runs, the unit may be overloaded or damaged.
- If the disposal hums but won’t turn, it may be jammed. Use the wrench slot on the disposal’s underside (if provided) to turn the impeller free—only with power off and following the manufacturer’s instructions.
4. Re-test with observation
- Restore power and run a short dishwasher drain while watching for the same problem.
- If the disposal now runs and drains normally, the issue was likely a blockage or loose hose. If not, note whether the unit trips, hums, or leaks—these observations will help a technician diagnose it.
What Not to Do
- Do not run the dishwasher if the disposal connection is incorrect; water backup risk increases.
- Do not stick your hand into the disposal while it is powered or before confirming the breaker is off.
- Do not try to force-remove a knockout plug while the disposal is mounted and powered—this can damage the unit or cause injury.
- Avoid running the dishwasher repeatedly if you see water backing into the sink; that can overflow or worsen a clog.
When to Call a Professional
Call a plumber or appliance technician if:
- You find the knockout plug is stuck and you’re not comfortable removing it safely.
- The disposal trips the breaker or reset button every time the dishwasher runs.
- There’s a leak at the hose connection or disposal that you cannot stop with a tightened clamp or simple seal replacement.
- If you notice persistent electrical smells, burning, or the motor makes unusual noises that you cannot clear.
If a leak caused the unit to stop and it won’t spin afterward, check the related guidance in Disposal won’t spin after leak. For persistent power/reset problems, see Disposal won’t reset.
Safety Notes
- Always shut power off at the breaker before working on or inside a disposal.
- Wear gloves and eye protection when inspecting under the sink; pipes and sharp metal are common hazards.
- Never use your hand to clear jammed blades unless you have fully disconnected power and are following the manufacturer’s instructions.
- If you are unsure about electrical checks, call a licensed technician—don’t improvise with live wiring.
Common Homeowner Questions
- Why does the disposal stop only when the dishwasher runs? The dishwasher pushes a short burst of water and food into the disposal; if there’s a blockage, a still-in-place knockout plug, or an electrical overload, that extra load can cause the disposal to stall or trip.
- Can I run the dishwasher if the disposal is disconnected? Not recommended. If the dishwasher is routed to the disposal inlet, leaving it disconnected or open can cause leaks or improper draining—correct the connection or get a plumber to reroute the drain.
- How do I reset the disposal? Turn off power, press the red reset button on the bottom of the unit, clear any visible jams while power is off, then restore power and test. If it won’t reset or keeps tripping, call a professional.