Quick Answer:
If odors vanish while you run water, the most likely cause is a dry or weakened trap seal, a slow drain, or sewer gases being temporarily pushed away by flow. Confirm the water level in the trap remains after use and inspect nearby fixtures and vents. Running water can mask the smell but is not a permanent fix.
Why This Happens
- Traps (the curved pipe under a drain) hold water to block sewer gas. If the trap is dry, cracked, or not sealing, gases reach the room until water flows and temporarily pushes them away.
- Slow-moving drains let organic material build up and produce odors that get stronger when water isn’t moving through and weaker while it flushes.
- Vent problems or a hardened P-trap can allow sewer air to enter; running water can change pressure briefly and stop the smell until pressure equalizes.
- Nearby fixtures or gaps in seals can create intermittent odors that disappear while a fixture is in use.
Step-by-Step What to Do
1. Observe and confirm the trap seal
Right after the smell appears, run enough water to refill the drain for 30–60 seconds. Then stop and watch the trap area (under the sink or access point) for several minutes. Confirm the water level in the trap remains after use. If the water drops or disappears, the trap is not holding a seal.
2. Check for leaks, cracks, or loose fittings
- Look for wet spots, drips, or damaged trap parts. A cracked or disconnected trap can allow odors even if water briefly covers it.
- Tighten connections by hand and, if comfortable, use basic tools to snug fittings. If you see corrosion or broken plastic, plan to replace that section.
3. Clear slow drains and organic buildup
- Use a drain catcher to remove visible debris from the sink. Follow with a plunger or a plumber’s snake to clear the line if draining slowly.
- Avoid frequent use of harsh chemical cleaners; use a baking soda and vinegar flush followed by hot water as a gentle cleaning step.
4. Check other fixtures and vents
- Run water in nearby sinks, showers, and floor drains to see if the smell moves or disappears similarly—this helps locate the source.
- If multiple fixtures are affected, suspect a vent or main sewer issue and consider professional help.
5. Short-term monitoring
- After repairs or cleaning, monitor the fixture for several days. Note if the odor returns only when not in use or persists.
- Keep a small log: when the smell appears, what you ran, and whether the trap stayed full.
What Not to Do
- Do not rely on running water as a permanent solution.
- Do not remove trap parts or the drain assembly if you are unsure—this can lead to system damage or allow sewer gas into cabinets and floors.
- Do not pour strong acids or bleach into traps regularly; these can damage pipes and harm you if misused.
When to Call a Professional
- Persistent odors after you confirm the trap holds water.
- Multiple fixtures smell at once, which could indicate a vent or main sewer problem.
- Visible cracks in traps or drainage parts, or if you’re uncomfortable making repairs.
- Any sign of sewage backflow or repeated clogs after basic cleaning.
Safety Notes
- Wear gloves and eye protection when working under sinks or handling drain debris.
- Avoid mixing cleaning chemicals (for example bleach and ammonia) — dangerous fumes can form.
- If you suspect sewer gas exposure (headache, dizziness, nausea), ventilate the area and leave immediately. Seek help if symptoms persist.
Common Homeowner Questions
- Why did the smell go away when I ran the tap?
Running water temporarily pushes air and odors through the trap and down the drain, masking the smell until the trap seal or source issue returns. - How can I tell if the trap is dry or leaking?
Visually inspect under the sink and watch the trap water level after use; any drop or wet leaks indicate a problem. - Could the smell come from a remodel or new fixtures?
If smells began after work, check seals and debris left in lines—see the guidance on Sink smells after remodel and also check for stagnant lines as described in Musty smell from drain.