Symptoms
- Dishwasher fills with dirty water from the sink side
- Food debris appears inside the dishwasher tub
- Problem happens mainly during the dishwasher's drain cycle
- Sink itself drains fine
Likely causes
- CommonClogged or partially blocked disposal chamberDebris sitting in the disposal restricts the dishwasher's drain path through it.
- CommonKnockout plug never removed from the dishwasher inletOn new or replaced disposals, this plug blocks the passage almost entirely if left in.
- PossibleClogged drain line or air gap after the disposalA partial clog downstream backs pressure into the dishwasher line.
- Less commonMissing or malfunctioning air gapSome codes require an air gap that, if clogged, causes backflow.
What to do
- Run the disposal with water to clear out any food debris sitting in the chamber.
- If this is a newer or recently swapped disposal, unplug it and check the dishwasher inlet nipple for a plastic knockout plug still in place; remove it with a screwdriver and hammer if needed.
- Check the air gap on the counter (the small cylindrical cap near the faucet) and clean out any debris inside it.
- Inspect and clear the drain hose between the disposal and the drain line for kinks or clogs.
- Run the dishwasher's drain or rinse cycle and watch the sink side disposal to confirm normal flow.
- If backup continues, treat it as a slow-draining disposal and check the trap as well.
Tools and parts
- Screwdriver
- Hammer (for knockout plug)
- Flashlight
When to call a pro
If clearing the knockout plug and air gap doesn't fix it, a plumber can check the full drain path for a deeper clog.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.