The Strange Smell Under the Sink That Isn’t Always a Plumbing Leak

I have opened enough under-sink cabinets to know they each have a personality. Some are tidy little supply closets. Some are a jumble of half-used sprays, mystery sponges, grocery bags, and one bottle of cleaner that has been standing there since the paint color was different.

Then there is the cabinet that smells “off.” Not always terrible. Just strange enough that you open the door, pause, lean in like a detective with questionable judgment, and think, “Please don’t be plumbing.”

Good news first: a weird smell under the sink does not automatically mean a leak. It can be moisture, trapped odor, old spills, stored products, drain gas, cabinet material, or something as unglamorous as a damp sponge quietly becoming a science project.

Start With the Smell, Not the Panic

The fastest way to solve an under-sink odor is to identify the type of smell before you start tightening pipes and blaming the P-trap. A musty smell usually points toward moisture, mildew, damp wood, or old water damage. A rotten-egg or sewer-like smell may involve the drain trap, venting, or an air admittance valve. A chemical smell often comes from stored cleaners, pest products, or something leaking from a container.

The EPA’s simplest mold guidance is still the most useful: indoor mold control starts with moisture control. In plain home-owner language, that means the smell may not come from an active leak, but it often means something under there has been staying damp longer than it should.

Before grabbing tools, empty the cabinet completely. Put everything on a towel, group similar items together, and give yourself a clear view of the cabinet floor, back wall, supply lines, drain parts, garbage disposal, shutoff valves, and any holes where pipes enter the wall.

5 Common Causes That Are Not Always a Leak

1. Damp cabinet material from an old drip

Particleboard and plywood can hold odor long after a leak has been fixed. If the cabinet floor is swollen, soft, darkened, bubbled, or smells strongest near one corner, you may be smelling old moisture trapped in the material.

Press gently with a gloved finger or screwdriver handle. If the base feels spongy, it may need drying, sealing, patching, or replacing—not just wiping.

2. Cleaning supplies that are fighting each other

Under the sink is a popular place to store cleaning products, but it is not always the best place for your nose. Leaky caps, damp rags, old scrub brushes, trash bags, dishwasher pods, and strong sprays can create a stale chemical stew.

Remove anything with a sticky bottle bottom, cracked cap, or strong smell. Never mix cleaners, especially bleach and ammonia-based products, and avoid storing products where small leaks or fumes can build up in a closed cabinet.

3. A dry or disturbed P-trap

The P-trap is the curved pipe under the sink that holds water and blocks sewer gases from coming back into the room. If a sink is rarely used, recently repaired, or draining oddly, that water seal can be low, disrupted, or siphoned out.

Run water for 30 seconds, then wait and sniff again later. If the smell fades, the trap may have been dry; if it returns quickly, you may have a venting, trap, or drain issue that deserves a plumber’s look.

4. Sludge in the drain, disposal, or overflow channel

Sometimes the cabinet smells bad because the drain above it is dirty, not because the plumbing below is leaking. Bathroom sink overflow channels can hold biofilm, toothpaste residue, and soap scum. Kitchen disposals can collect food residue under the splash guard.

Clean the drain opening, disposal splash guard, stopper, and overflow area if your sink has one. This is not glamorous work, but neither is pretending the smell is “probably fine” for six months.

5. Condensation, not dripping

Cold water pipes can sweat in humid rooms, especially in summer or in poorly ventilated cabinets. The moisture may look like a leak, but it is really condensation forming on the pipe and dripping onto the cabinet floor.

Dry the area completely, then run cold water and watch the supply lines. If tiny beads form evenly along the pipe instead of at a joint, insulation sleeves and better airflow may help.

A Simple Under-Sink Smell Check You Can Do Today

1. Clear and separate everything

Take out every item and smell them individually away from the cabinet. I know this sounds ridiculous, but I have solved more than one “plumbing smell” by discovering a damp rag, old sponge, leaking dish soap, or forgotten trash bag roll that smelled like regret.

Throw away anything damp, sticky, expired, or questionable. Wash storage bins before putting them back.

2. Do the dry-towel test

Wipe the cabinet floor, pipes, valves, disposal, and drain connections dry. Lay a clean paper towel or piece of cardboard under the plumbing and leave it for several hours, then check for new spots.

For a stronger test, run water in both hot and cold, fill the sink partly, then drain it while watching the P-trap and connections. A leak often shows up only when the drain is under a little pressure.

3. Sniff by zone

Smell near the cabinet floor, back wall, drain pipe, supply valves, disposal, and stored items. The goal is to locate the strongest odor, not simply confirm that the whole cabinet smells unpleasant.

If the strongest odor is near a wall opening, look for gaps where pipe penetrations meet the cabinet back. Sometimes odor travels from a wall cavity, pest activity, or an unfinished opening rather than the sink parts themselves.

4. Check for pests without turning it into a horror film

Look for droppings, shredded paper, gnaw marks, dead insects, or nesting material. Under-sink cabinets offer warmth, darkness, water access, and crumbs from trash or recycling bins nearby.

Use gloves, avoid stirring up dust, and call a pest professional if you see clear signs of rodents or a recurring infestation.

5. Let the cabinet breathe

After cleaning, leave the doors open for several hours with good ventilation. A small fan aimed nearby—not directly into suspicious mold or dust—can help dry the space.

If the smell improves only when the cabinet is open, the problem may be trapped humidity, stored products, or odor-soaked cabinet material.

What to Clean, What to Replace, and When to Call a Pro

For light mustiness, start with cleaning and drying. Use mild soap and water on washable surfaces, then dry thoroughly. The CDC notes that mold grows where moisture is present, including around leaks or damp materials, so cleaning without fixing the moisture source is usually a short vacation from the smell, not a cure.

Replace saturated shelf liners, moldy cardboard, damp rags, and swollen cabinet panels that keep smelling after cleaning. If the cabinet floor is damaged but structurally sound, a removable waterproof mat can help protect it after the source is solved.

Call a plumber or qualified pro if you notice any of these:

• Active dripping or wet drywall
• Sewer smell that returns after running water
• Gurgling drains, slow draining, or bubbling toilets
• A loose, missing, or incorrectly shaped trap
• Strong rotten-egg odor that could suggest gas or sewer gas
• Mold covering a large area or affecting porous materials

If you ever smell natural gas near a sink, stove, water heater, or utility area, leave the home and contact your gas utility or emergency services from outside. That is not a “clean the cabinet and see” situation.

A Fresh Cabinet Is a Very Satisfying Little Win

A strange smell under the sink can feel ominous, but most causes become easier to handle once the cabinet is empty, dry, and visible. The trick is to investigate in layers: stored items first, moisture second, drain odor third, cabinet material fourth, and pests or wall-cavity issues if the obvious suspects do not explain it.

I like this repair because it rewards patience more than brute force. You do not need to tear anything apart on day one. You need a towel, a good light, a careful nose, and the willingness to evict the damp sponge that has been causing drama from the back corner.

Once the source is handled, keep the cabinet simple: fewer products, dry storage bins, a washable liner, and a quick monthly look at the pipes. It is not fancy maintenance, but it is the kind that keeps a small smell from turning into a bigger home problem.