Checklist • Updated Jan 25, 2026 • 12 min read

Under-sink smell checklist: P-trap, disposal, and hidden leaks

Under-sink smells usually come from moisture, food residue, or a dry P-trap. This checklist helps you narrow it down without risky DIY.

Michael Rivera

Michael has inspected thousands of under-sink cabinets during his 12+ years in property maintenance. Most smells have a simple explanation—and a simple fix.

Worth noting: This checklist is about identifying the source, not doing plumbing repairs. If you find active leaks, persistent sewer gas, or mold on porous surfaces, stop and contact maintenance right away.

Quick guide: If you smell sewer gas, start with the P-trap (dry trap is common). If you smell musty, look for moisture/leaks. If it’s sour, the disposal or trash is the usual culprit.

Under-sink plumbing showing P-trap and drain pipes — Under the sink: the curved P-trap holds water to block sewer gases. Check here first for leaks or dry traps.

Smell types (quick identification)

Smell	Often means	First action
Sewer / “rotten egg”	Dry trap, venting issue, or leak	Run water 30–60 sec in that fixture, then re-check
Musty / damp	Moisture, hidden leak, wet cabinet	Dry everything, take photos, check for return
Sour / rotting food	Disposal residue, dirty splash guard	Clean guard + hot water + dish soap flush
Trashy / stale	Bin/recycling area	Wash bin, replace liner, ventilate

Fast sniff test (where is it strongest?)

Cabinet wood / floor: musty smell often means moisture.
Drain area: sewer-ish smell can mean P-trap issues.
Garbage disposal: sour/rotting smell is usually residue.

Tools & supplies

Flashlight (leaks hide in shadows).
Paper towels (to check “is it actively wet?”).
Dish soap + warm water.
Microfiber cloth (drying helps you detect return moisture).
Trash bag (for dirty liners / food residue cleanup).

P-trap basics (why it matters)

The P-trap is designed to hold water. That water seal blocks sewer gases from entering your cabinet. If the trap dries out (rarely used sink, vacation, or a small leak), odor can rise from the drain.

Checklist

Check	What you see/smell	Next action
Look for moisture	Damp cabinet, swelling, dark stains	Photo + report to maintenance
P-trap area	Sewer smell near pipes	Run water 30–60 sec; smell improves = trap was dry
Disposal splash guard	Sour smell, slime	Clean guard + flush with dish soap and water
Trash can / recycling	Smell concentrated there	Wash bin; use liners; ventilate

Step-by-step (low-risk)

1) Check for hidden moisture

Remove everything from under the sink.
Dry the cabinet completely (floor and walls).
Wrap a dry paper towel around each visible connection (P-trap joints, shutoff valves) and wait 2–3 minutes.
Look for damp spots. If any connection wets the towel, it’s a maintenance ticket.

1b) Overnight leak test (easy, high signal)

Line the cabinet floor with dry paper towels (or a piece of cardboard).
Don’t use the sink for a few hours (or overnight).
Check in the morning: even a slow drip will show up clearly.
If the towels/cardboard are damp, take photos and file a maintenance request.

2) Rule out a dry trap

Run cold water for 30 seconds, then hot for 30 seconds.
Wait 10 minutes and re-check the smell.
If the smell returns quickly (same day), there may be a vent/seal issue: call maintenance.

2b) Where sewer smells usually come from

Dry P-trap (most common): smell improves after running water.
Loose connection: sewer smell + occasional dampness.
Venting issue: gurgling sounds + sewer smell that persists.

3) Clean disposal/splash guard residue

Safety: keep the disposal off; don’t put hands into the unit.
Scrub the underside of the rubber splash guard with dish soap.
Flush with hot tap water + dish soap for 60–90 seconds.

3b) Deeper disposal odor routine (still renter-safe)

Dish soap + hot water flush first (often enough).
Ice + salt (optional): some people use it to knock residue loose. If you’re unsure, skip it—cleaning the guard + flushing is safer and usually sufficient.
Don’t pour grease down the drain. It coats pipes and traps food residue that smells later.

4) Check the trash/recycling zone

Wash the bin and let it dry fully.
Replace liners and keep the cabinet slightly cracked for airflow (if possible).

Common leak points (quick visual scan)

Shutoff valves (small drips at the base can wet the cabinet over time).
P-trap joints (especially after someone bumped the pipe while storing items).
Disposal connection (where the drain line meets the disposal).
Dishwasher hose connection (if present).

Dishwasher clue (if you have one)

If the smell appears after running the dishwasher, check under the sink afterward. A small drip from the dishwasher hose connection can create a musty cabinet quickly.

Prevention (2-minute routine)

Keep the cabinet dry: wipe small spills immediately.
Run the faucet weekly in rarely used sinks to keep the P-trap wet.
Don’t store loose chemicals that can spill and create lingering odors.
Scrape plates before using the sink/disposal—less residue, less smell.

If odors return quickly after cleaning, it’s often moisture, not “dirty dishes.” Focus on finding the source.

When to call maintenance (and what to say)

Moisture returns after drying (hidden leak is likely).
Persistent sewer smell even after running water.
Any mold growth on porous cabinet material.
Soft/swollen wood (water damage progresses fast).

Maintenance request template (copy/paste)

Subject: Under-sink odor / possible leak inspection

Hi [Landlord/Maintenance], there’s a persistent odor under the [kitchen/bathroom] sink. I checked for obvious issues and noticed [sewer smell near the P-trap / musty odor + damp cabinet / possible moisture at a connection]. Could you please inspect for a leak or vent/trap issue and repair as needed? I can share photos and I’m available [times]. Thank you.

FAQ

Is a sewer smell dangerous? It can indicate sewer gas entering the space. If it persists after running water, report it.
Should I pour bleach down the drain? Avoid harsh chemicals in rentals; it can damage finishes and won’t fix a trap/vent/leak issue.
Why does it smell worse at night? Odors are more noticeable when the home is closed up and airflow is lower.

Optional: photo checklist (for your ticket)

Wide shot under the sink (shows the layout).
Close-up of any damp connection or stain.
Paper towel test photo if it shows moisture.
Timeline note: when you smell it and whether it changes after running water.

Final sanity checks

Rarely-used sink? Run water weekly so the P-trap doesn’t dry out.
Smell changes after running water? That points to a dry trap more than “dirty cabinet.”
Odor + any dampness is a faster maintenance ticket than repeated cleaning.
If you’re unsure, take photos and send a short note—catching a small leak early protects your deposit.

If you want one simple rule: smell + moisture = maintenance. Cleaning only helps if the source is residue/trash—leaks and venting issues come back.

Stop signs (call maintenance)

Any active drip or wetness that returns after drying.
Strong sewer smell that persists after running water.
Visible mold growth on porous cabinet material.