Quick Answer:

If your bathroom sink gives off a strong sewer smell after not being used overnight, the most likely cause is a dry or partially emptied trap or a blocked overflow or vent letting sewer gas into the room. Start by running water to refill the trap, flush the overflow channel, and check the vent for a draft. These simple checks often stop the smell quickly.

Why This Happens

Plumbing fixtures use a water-filled P-trap to block sewer gas. If the sink sits unused, that water can evaporate or leak out, removing the seal and allowing odors to come up through the drain. Other common causes:

• Evaporation or a leaking trap.
• A clogged or dirty overflow channel that allows odor to bypass the trap.
• Venting problems that let sewer gas travel into the house instead of out through the roof.
• Intermittent plumbing interactions — for example, a toilet or other drain affecting the sink; if you see noise or behavior patterns, see Bathroom Sink Gurgles When Toilet Is Flushed.

Step-by-Step What to Do

1. Run water to refill the trap

Turn on the sink and let both hot and cold run for 30–60 seconds so the P-trap refills fully. If the smell goes away, the trap seal was likely the problem.

2. Check for leaks and slow drainage

• Look under the sink for any dampness or drips around the trap and tailpiece. Tighten slip nuts if you see small leaks.
• If drainage is slow or the smell returns after running hot water, note the pattern — it can indicate a partial clog or buildup. For issues tied to temperature, see Bathroom Sink Drains Slow Only With Hot Water.

3. Flush the overflow channel

Overflow channels can collect grime and let odors bypass the trap. To flush it:

• Pour a bucket of warm water into the overflow opening (usually near the back of the basin) to dislodge trapped debris.
• Follow with running the sink normally to clear anything loosened.

4. Inspect the vent for draft

A blocked or damaged vent can change airflow and let sewer gas into the home. You can do a basic check:

• With someone else running a large fixture (like a tub) nearby, check whether the sink smell worsens or whether you can feel a draft at the drain openings. That suggests a venting issue.
• If your roof is safely accessible and you are comfortable, a quick visual check of the vent pipe opening for obvious blockages (leaves, bird nests) can help. If you’re not comfortable on the roof, skip this and consider calling a pro.

5. Test and repeat

• After each step, leave the sink unused for a few hours or overnight to see if the smell returns.
• If the smell temporarily disappears but comes back, that’s a sign of a recurring leak, vent problem, or deeper clog.

What Not to Do

• Do not pour bleach or concentrated chemicals directly into the overflow — it can damage finishes and plumbing components and won’t reach the vent or trap seal effectively.
• Do not block or seal vent pipes yourself in an attempt to stop odors — vents are needed for safe plumbing function.
• Do not use open flames or test odors with matches; sewer gas can be hazardous in high concentrations.
• Avoid excessive use of liquid chemical drain cleaners; they can harm pipes and won’t fix venting or trap problems.

When to Call a Professional

• If the smell persists after you refill the trap, flush the overflow, and have inspected for visible leaks.
• If you find or suspect a blocked vent (roof access required) or a recurring leak inside walls or under the floor.
• If you notice gurgling drains, sewage backups, or multiple fixtures smelling at once — these often mean a more serious venting or sewer-line issue.
• If you are uncomfortable working on roof vents or removing traps safely.

Safety Notes

• Ventilate the room by opening windows and doors while diagnosing odors.
• Avoid prolonged inhalation of sewer gas; brief exposure is usually unpleasant but extended exposure can be harmful.
• Use gloves when checking traps. Place a bucket under the trap before loosening fittings to catch water and debris.
• If you suspect a major sewer leak or backup, leave the area and call a professional immediately.

Common Homeowner Questions

• Why does the smell vanish after I run the water? Running water refills the P-trap, restoring the water seal that blocks sewer gas.
• Can a blocked vent cause odors only at night? Yes — pressure changes and temperature shifts overnight can make venting problems more noticeable when fixtures sit unused.
• Will pouring bleach down the sink fix sewer smell? No. Bleach may mask odors temporarily and can damage parts; it won’t restore a dry or leaking trap or clear vent issues.