Likely cause: a stopper that no longer seals, or is missing

Why this happens

Family Handyman says a twist-and-drop style stopper needs its O-ring replaced from time to time, and, about push-in style strainers with a plastic knob or a nonreplaceable neoprene stopper, that the plastic parts can break and can lose their sealing ability if exposed to boiling water, which is why it says to avoid that style when you buy. Kohler lists its strainer basket as a part sold separately from the drain body.

How to fix it

1. Check the cabinet under the sink before you add any water. This page is for a bowl that loses water down the drain while the cabinet stays dry. If anything under the sink is wet or dripping, do not add water and use the kitchen sink guide again, then choose the wet cabinet answer. If a plug, cord or outlet under the sink is wet, do not touch it and call an electrician. Turn off the breaker only if you can reach the panel safely. If you cannot, stay away from the area and call an electrician or emergency services.
2. Keep the faucet off. If this bowl has a garbage disposal, shut off its power first, by unplugging it or turning off its breaker, as Waste King describes before removing a stopper. If the disposal has no wall switch and starts when a stopper is set in the opening, Waste King calls that a batch feed type, so keep its power off for the whole job. Keep your fingers out of the opening. Waste King says never to place your hand inside the disposal.
3. Lift the stopper or basket out. Do not force it. If it is stuck, stop and call a plumber.
4. Look at the rubber. A seal or O-ring that is torn, flattened or missing, or a stopper that is gone, will let water drain away. If the rubber looks sound and the stopper still will not hold water, the cause may be something this page cannot check, so stop and call a plumber instead of buying parts to guess.
5. On a bowl with no disposal, keep the old basket or stopper and take it with you so you can match it. Family Handyman says a twist-and-drop style needs its O-ring replaced from time to time. Elkay says drain bodies and baskets vary by model, so check the maker's instructions or parts list for the exact part, and fit it by those instructions.
6. If this bowl has a garbage disposal, stop here and call a plumber instead of buying parts. This page cannot check how a stopper seats in a disposal's flange, and Bob Vila says that resealing a disposal's sink flange means detaching the disposal first, which is plumber's work. On a bowl without a disposal, if no matching basket or O-ring is available, the next step is a new strainer assembly, which Family Handyman rates as a beginner job. That job means taking the strainer apart from under the sink, and Family Handyman's guide to replacing a basket strainer shows the steps. Shut off the water supply first, as Kohler's installation guide says, and keep the faucet off. If a shutoff will not turn, if the locknut will not turn, or if a chemical drain cleaner has gone down this sink, do not go on and call a plumber instead. Never force a shutoff.
7. Put the new part in, fill the bowl a few inches and watch the water level for several minutes.

When to stop. If a new stopper or basket seats properly and the bowl still empties, or the drain body looks cracked, stop and call a plumber.

Start this troubleshooter over

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