Quick Answer:
A sour, rotten smell coming from the sink drain usually means trapped food, grease build-up, a dry or partially blocked P-trap, or something rotting under the cabinet. Start by running hot water for 30–60 seconds, then sniff near the drain opening vs. the overflow to isolate the source. Flush with a kettle-sized amount of hot (not boiling) water and check that the P-trap has water by listening for a hollow gurgle when you pour a cup in. If the smell continues or you have repeated backups, call a plumber.
Why This Happens
- Food scraps and grease cling to the sides of the drain or the garbage disposal and produce a sour, rotten smell as they break down. If the disposal is the source, the problem may be similar to Grease odor from disposal.
- A dry or damaged P-trap can let sewer gas into the kitchen. A partial clog can trap organic matter and let smells escape when water moves through.
- Leaks or spoiled food under the sink create persistent, musty odors—this is often described as a Rotten food smell under sink.
- Bacterial film (biofilm) builds up in slow drains and produces a sour, persistent odor.
Step-by-Step What to Do
1. Isolate the source
Run hot water for 30–60 seconds, then sniff near the drain opening vs. the overflow to isolate the source. Smelling stronger at the overflow or around the sink edge points to residue in the sink or overflow channel. Smell concentrated at the drain or in the cabinet points to the trap, disposal, or a leak.
2. Flush with hot water
Boil water only if you know the pipes are metal; otherwise use a kettle-sized amount of hot (not boiling) water and pour it slowly down the drain. This helps melt grease and flush loose debris. Repeat once or twice and check whether the odor fades.
3. Check the P-trap
Check that the P-trap has water by listening for a hollow gurgle when you pour a cup in. If the trap is dry, pour a cup of water into the drain to refill it—this restores the seal against sewer gas. If you hear gurgling or the trap empties quickly, there may be a leak or venting issue.
4. Clean the disposal and visible parts
Turn the disposal off and disconnect power before reaching in. Use tongs or a brush to remove visible food particles from the disposal and stopper. Scrub the stopper, sink drain opening, and the sink overflow slot with a small brush and mild dish soap. A short run of cold water with the disposal on can help push remaining particles through.
5. Inspect under the sink
Open the cabinet and look for spoiled food, leaks, or standing water. Wipe up any spills and replace anything that has soaked and begun to rot. If you suspect something stuck deeper in the drain or a damaged trap, stop and consider the next step.
6. Re-check and monitor
After cleaning and flushing, run the faucet and smell again. If the odor returns within a day or you get backups, the issue likely needs professional attention.
What Not to Do
- Don’t pour harsh chemicals into the drain “to kill the smell” if the sink is draining slowly—backed-up chemicals can damage fittings and create dangerous fumes.
- Don’t mix cleaning chemicals (for example, bleach and ammonia) — that can create toxic gas.
- Don’t stick your hand into a running or recently used garbage disposal. Always disconnect power first.
- If the odor comes with repeated backups or strong sewer-gas smells, a plumber is appropriate rather than relying on over-the-counter chemical fixes.
When to Call a Professional
- Persistent odor after cleaning and flushing.
- Repeated backups or slow drainage that returns quickly.
- Strong sewer-gas smell (rotten-egg or methane-like) or smell from multiple drains—this can indicate a venting or sewer line problem.
- Visible leaks under the sink, damaged trap, or signs of pipe corrosion.
Safety Notes
- Use hot water that is hot but not boiling on PVC or plastic fittings to avoid warping. Boiling water is safer only for metal pipes.
- Wear gloves and eye protection when cleaning drains or the disposal. Use tools (tongs, brushes) instead of your hands.
- Turn off power to the disposal at the breaker or unplug it before working on it.
- Be cautious with home remedies—baking soda and vinegar are safe in small amounts but won’t fix deep clogs. Never combine them with bleach or other strong chemicals.
Common Homeowner Questions
- Why does the smell appear only when I run water? Running water loosens trapped food and moves sewer gas; the flow can push trapped fumes up the drain so you notice the smell.
- Is the garbage disposal the likely cause? Often yes—food and grease collect there and in the splash area; cleaning the disposal and running hot water usually helps.
- Will boiling water fix the problem? Not always. A kettle-sized amount of hot (not boiling) water can help dissolve grease, but persistent smells usually need cleaning of the trap, disposal, or inspection for leaks.
For more related articles, see the Food Waste Odors & Bio-Buildup hub.