Quick Answer:
If you notice sewer-like odors coming from the overflow opening after water runs out, start by smelling the overflow opening right after a flush and then again after a short rinse. If the smell returns quickly, the cause is often a trap or overflow channel problem rather than a distant sewer backup, but persistent odor can indicate a venting or sewer connection issue that needs professional attention.
Why This Happens
- Dry or partially emptied trap: The P-trap under a sink or tub holds water that blocks sewer gas. If it’s siphoned out or not refilled, gas can reach the overflow channel and the room.
- Debris or biofilm in the overflow channel: Gunk inside the overflow path can trap old water that smells, and flushing can force the odor out.
- Plumbing vent or sewer line problems: If the vent is blocked or the main sewer has partial blockage, pressure changes can pull sewer gas into fixtures through overflow openings.
- Leaky or faulty internal seals: Cracked or missing seals around the overflow and internal fittings can let sewer air through even when traps are present.
Step-by-Step What to Do
1. Smell the overflow opening after flushing
Flush or run the fixture and put your nose near the overflow opening (without touching). Note whether you can smell sewer odor immediately.
2. Rinse and check how quickly the odor returns
Rinse the overflow and the fixture with clean water for a minute, then sniff again. If the odor returns quickly after rinsing, that suggests trapped gunk or a vent/trap issue rather than a one-time stale smell.
3. Clean the overflow channel
Use a soft bottle brush or a flexible cleaning tool and warm soapy water to scrub into the overflow opening. Remove any visible sludge. Repeat the rinse and smell test.
4. Refill the trap
Run water in the sink, tub, or fixture for a minute to make sure the P-trap is full. If the trap was dry, this often stops sewer gas from coming up the overflow.
5. Inspect for constant dripping or internal leaks
Look for steady dripping into the overflow or from internal fittings. A continuous leak can keep the overflow channel wet and smelly. Check guides like Overflow hole dripping constantly if you see persistent dripping.
6. Check other fixtures
Smell other drains and overflows. If more than one fixture smells, the cause may be a vent block or main sewer problem—see troubleshooting notes such as Sink overflows despite clear drain for similar symptoms.
7. Test for gurgling or slow drainage
Listen for gurgling when other fixtures run. Slow drainage, gurgling or sewer odors in multiple places are signs to escalate.
8. Call a professional if it persists
If cleaning, refilling the trap, and rinsing do not stop the smell—or if you see sewage backup—contact a licensed plumber. Persistent odor after these checks often needs a camera inspection or vent/sewer evaluation.
What Not to Do
- Do not dismiss sewer-like odor; if it persists after flushing, professional help may be appropriate.
- Do not pour large amounts of bleach or harsh chemicals down the overflow in hopes of fixing the smell—these can damage seals and plumbing and create dangerous fumes when mixed with other cleaners.
- Do not attempt major drain removal or cutting into pipes unless you have the right tools and experience—this can make the problem worse.
When to Call a Professional
- The odor remains after cleaning, flushing, and refilling the trap.
- Multiple fixtures smell or you notice gurgling, frequent backups, or sewage in the fixture.
- You suspect a broken or missing trap, damaged seals, or a blocked vent stack—these require tools and experience to diagnose.
- Any time you’re unsure or the smell is strong and persistent; better safe than risking prolonged exposure or hidden damage.
Safety Notes
- Ventilate the area while you work—open windows and run fans.
- Wear gloves and eye protection when cleaning drains or using mild cleaners.
- Avoid mixing cleaners. Never mix bleach with ammonia or acidic products.
- Prolonged exposure to sewer gas can cause headaches, nausea, and irritation—if you feel ill, leave the area and seek help.
Common Homeowner Questions
- Why did the smell start after I used the fixture? Often flushing moves trapped gunk or forces sewer gas through a compromised trap or overflow; a quick rinse and trap refill can diagnose this.
- Can I fix it myself? Many times yes—clean the overflow, refill the trap, and rinse. If the smell persists or appears in multiple places, call a plumber.
- Is sewer gas dangerous? Short exposures usually cause irritation; strong or continuous smells should be addressed promptly for health and safety.