Quick Answer:

If the sink or air gap spits water only when the dishwasher drains, the most likely causes are a bad drain hose installation (no high loop or air gap), a partial clog in the branch drain that takes the dishwasher runoff, or the garbage disposal inlet has a knockout plug or obstruction. Start by checking the hose routing and air gap, clean the air gap, then test a drain cycle with a bowl under the trap to confirm a partial clog before attempting repairs.

Why This Happens

• Dishwasher drain water is routed into the sink drain or garbage disposal. If that downstream path is partly blocked, the dishwasher can push water back into the sink or air gap when it tries to drain.
• If the drain hose is not looped up under the countertop or an air gap is clogged, sink water or disposal water can flow back into the dishwasher line during the sink or disposer use.
• A disposal inlet may still have a factory knockout plug or be blocked by food buildup, which stops dishwasher water from entering the disposer and forces it back into the sink.
• If you see repeated slow drainage from multiple fixtures, it can be a partial clog in the branch drain feeding that sink—this is a common sign and relates to ongoing blockages like the one described in Drain Keeps Clogging Every Few Days.

Step-by-Step What to Do

1. Prepare and protect the area

• Place towels on the cabinet floor and have a bucket handy.
• Turn off the dishwasher’s power at the breaker if you will access the dishwasher or disposal. Turn off the hot and cold supply valves if you will remove the trap. Never remove hoses without first shutting power/water.

2. Check the drain hose loop (high loop)

• Open the cabinet under the sink and find the dishwasher drain hose. It should loop up and be secured high under the countertop before dropping to the sink or disposal. This high loop prevents sink water from running back into the dishwasher.
• If the hose runs directly down without a high loop, secure it up under the countertop or use the dishwasher bracket to create the loop and retest.

3. Clean the air gap (if present)

• Air gaps sit on the sink rim or countertop. Remove the decorative cap and clean inside of food and debris. Rinse with water and run a short dishwasher drain cycle to see if that clears the backflow.
• If there is no air gap, make sure the high loop is present and tight to prevent backflow.

4. Inspect the garbage disposal inlet for a knockout plug

• Look where the dishwasher drain hose connects to the disposal: if the disposal was newly installed or never had the knockout removed, a round plug may still be inside. Do not try to remove that plug while the disposal is connected and powered.
• If you can’t see clearly, shine a flashlight into the inlet. If the inlet appears blocked or the hose connection is obstructed, stop and plan to remove the disposal or have a pro handle it safely. If you want to read more about similar disposal-related issues, see Drain Works Until You Use the Garbage Disposal.

5. Test the drain cycle with a bowl under the trap

• Put a shallow bowl or pan under the sink trap to catch any water. Run a dishwasher drain (start a short or rinse cycle) and watch where the water goes.
• If water spills out of the sink tailpiece or the trap fills and then backs up into the bowl, you likely have a partial clog in the branch line that the dishwasher is forcing against.

6. Verify and clear a partial clog in the branch line

• If the test shows slow drainage or backup, try a sink plunger on the sink (block the other basin if you have a double sink). If that does not help, place the bucket under the trap, remove the trap to inspect and clean trapped debris, then test again.
• If the obstruction is deeper in the branch line, use a hand-crank drain snake from the sink side or call a plumber. Avoid pouring chemical drain cleaners—they can damage pipes and are dangerous if they contact dishwasher components.

What Not to Do

• Do not force the dishwasher to keep draining (flood risk); unplugging or restarting it while it is backing up can overflow the sink or floor.
• Don’t remove hoses without shutting power/water.
• Don’t pour strong chemical drain cleaners into the drain while the dishwasher is connected—these can damage seals and are hazardous if the dishwasher drains those chemicals into its components.
• Don’t attempt electrical or disposal motor work while the appliance is powered—always shut off power first or call a professional.

When to Call a Professional

• Repeated backups after you clear the trap or snake the sink.
• You find the knockout plug but are unfamiliar with removing it safely or the disposal needs to be detached to remove it.
• There is a strong sewer smell, multiple fixtures backing up, or water reaches the floor. These signs point to a deeper clog or sewer-line issue.
• If electrical work is needed (garbage disposal replacement or wiring issues) or you are uncomfortable with the steps above, call a licensed plumber or appliance technician.

Safety Notes

• Always turn off power at the breaker before working on the dishwasher or disposal. Confirm power is off with the appliance controls.
• Shut off water supply valves before removing drain traps or hose fittings. Keep a bucket and towels ready.
• Avoid chemical drain cleaners when a dishwasher or disposal is involved; they can cause burns and damage parts.
• If you must remove the knockout plug, ensure the disposal is disconnected and follow the manufacturer’s instructions or hire a pro.

Common Homeowner Questions

• Why does the sink only back up when the dishwasher runs?
  Because the dishwasher pushes a concentrated volume of water into the same drain path; a partial clog, lack of high loop/air gap, or a blocked disposal inlet will cause backup under that load.
• Can I fix this myself?
  Yes for simple steps: check the hose loop, clean the air gap, clear the trap, and plunge or snake the sink. Stop and call a plumber for knockout removal, deep clogs, or electrical work.
• Will running the dishwasher empty the clog?
  No—running the dishwasher can push water into the sink and cause flooding. Use targeted clearing (trap removal, snake) or call a pro instead.