A rotten-egg or sulfur smell indoors usually means sewer gas is escaping into your home — most often from a dry drain trap. But first, an important safety check.
Written bySteven Lockhart·Reviewed byMiguel Lopez, Owner & Licensed Plumber, New Pro Plumbing (CSLB #974600)·Last reviewedAugust 2026 · 8 min readQuick Answer
A sewer-gas smell indoors — like rotten eggs or sulfur — usually comes from a dry P-trap, a venting problem, or a drain leak letting sewer gases into the home. First, an important safety check: if the smell could be natural gas, especially near gas appliances or with hissing, treat it as a gas leak, leave, and call 911. Otherwise, New Pro finds and fixes the source.
Both sewer gas and natural gas can smell like rotten eggs — natural gas is deliberately odorized with a sulfur-smelling additive so leaks are noticeable. So before anything else: if the smell is strongest near a gas appliance, you hear hissing, or anyone feels dizzy or nauseated, treat it as a possible gas leak. Leave the building and call 911 and your gas company from outside.
Sewer gas smell in house — Sewer gas smell repair is the diagnosis and correction of sewer gases entering a home — after ruling out a natural gas leak — usually from a dry P-trap, a venting problem, or a drain leak.
If a gas leak is ruled out, the rotten-egg smell is sewer gas escaping into the home, and finding its source is a straightforward job for an emergency plumber.
The most common cause — a P-trap in an unused drain dries out and lets sewer gas rise. Running water refills it.
A blocked vent or a cracked drain or wax ring can also let sewer gas in; a strong sewage smell with backing-up drains is a sewage backup, not just odor.
Rule out gas first, then track down the sewer-gas source.
If it could be a gas leak, leave and call 911 first — do not investigate.
Guest baths, floor drains, and rarely used fixtures dry out first.
Run water in each to refill the P-trap and reseal it.
Where it is strongest points to the source.
A blocked vent, cracked drain, or bad toilet wax ring can leak gas.
For a persistent smell, we find and fix the source.
Could it be natural gas? Natural gas is odorized to smell like rotten eggs too. If the smell is near a gas appliance, or you hear hissing, treat it as a gas leak — leave and call 911. See what to do in a gas leak.
Persistent rotten-egg smell and it is not gas? We will find the source — call us.
Get a Free QuoteYour plumbing has two features whose entire job is to keep sewer gas out of the house: traps and vents. Every drain has a P-trap, the U-shaped bend that holds a little water to form a seal that gases cannot pass. And the vent system — pipes running up through the roof — lets air in so drains flow smoothly and lets sewer gases escape harmlessly above the house rather than into it. When a smell gets indoors, one of these two systems has usually been defeated.
The most common defeat is simple evaporation. A trap in a guest bathroom, a floor drain, or a rarely used fixture slowly loses its water to evaporation, and once the seal is gone, sewer gas rises straight into the room. Running water for a minute refills it. A blocked vent is subtler — it can pull the water out of traps by suction as other fixtures drain, which is why a venting problem sometimes shows up as multiple smelly drains and gurgling, and can point to a main line issue. The U.S. CPSC’s home-safety guidance is a useful reference on keeping household air safe, and a properly sealed plumbing system is part of that.
For a smell that a little water does not fix, we track down the real source — a vent, a cracked drain, or a failed seal — and repair it as licensed work you can verify with the CSLB (#974600), permitting anything that requires it through the Los Angeles Department of Building & Safety. The one thing we always put first, though, is ruling out that the rotten-egg smell is natural gas rather than sewer gas.
Whether refilling the trap fixes it tells you which.
| Clue | Likely cause | Fix |
|---|---|---|
| Unused-drain smell | Dry P-trap | Run water to refill |
| Constant smell | Venting problem | Clear / repair vent |
| Near a toilet | Bad wax ring / crack | Reseal / repair |
Whatever the source, we find it and seal it — flat-rate, the way 24/7 emergency plumbing should be.
If it is a dry trap, the fix is free — just run water. Beyond that, cost depends on the source: resealing a toilet, clearing or repairing a vent, or fixing a cracked drain each vary in effort. We locate the exact source and quote it flat-rate, so a simple fix stays simple.
We find the real source before quoting — see how our emergency plumbing service prices flat-rate.
A homeowner notices a rotten-egg smell in a guest bathroom that is rarely used.
New Pro rules out gas, finds the seldom-used shower's P-trap had dried out, and refills and treats it.
The smell is gone, and the homeowner learns to run that drain periodically to keep the trap sealed.
The Bottom Line
A rotten-egg smell indoors deserves one quick safety question first — could it be natural gas? If so, leave and call 911. If not, it is sewer gas, and the cause is usually a dry trap you can refill in seconds, or a venting or drain issue we can find and seal.
It can be either, which is why you should rule out a natural gas leak first. Natural gas is odorless, so utilities add a sulfur-smelling odorant that smells like rotten eggs — the same as sewer gas. If the smell is near a gas appliance, you hear hissing, or anyone feels dizzy, headachy, or nauseated, treat it as a gas leak: leave the building and call 911 and your gas company from outside. If a gas leak is ruled out, the smell is sewer gas from your plumbing.
Once a gas leak is ruled out, the most common cause is a dry P-trap — the curved pipe under a drain holds water that blocks sewer gas, and in a rarely used drain that water evaporates, letting gas rise into the room. Other causes include a blocked or improperly installed plumbing vent, a cracked drain pipe, or a failed wax ring under a toilet. Locating where the smell is strongest usually points to the source.
If it is a dry trap, simply running water down that drain for a minute refills the P-trap and reseals it against sewer gas — this fixes many cases instantly, especially in guest bathrooms and floor drains. Pouring a little water into rarely used drains periodically prevents it. If refilling traps does not help and the smell persists, the cause is likely venting or a drain leak, which needs professional diagnosis rather than a home remedy.
Sewer gas is mostly a mix that includes hydrogen sulfide (the rotten-egg smell) and methane. At the low levels of a typical household smell it is more unpleasant than immediately dangerous, but it should not be ignored — higher concentrations can be harmful, and methane is flammable. Persistent or strong sewer-gas smells warrant finding and fixing the source. And because the smell overlaps with natural gas, any doubt about which it is should be treated as a gas leak.
Intermittent sewer smells often point to a dry trap in a fixture you use infrequently — the smell appears after the trap water has evaporated and disappears once you run water again. It can also come and go with changes in air pressure or wind affecting the plumbing vents, or when a particular fixture is used. If it recurs in the same spot, that fixture’s trap or vent is the place to look, and we can confirm the source.
Written by the New Pro Plumbing content team and reviewed for technical accuracy by Miguel Lopez, owner and California-licensed plumber (CSLB #974600). Guidance follows U.S. EPA, CSLB, and LADBS sources where cited. Any figures reflect New Pro Plumbing's flat-rate practice and West LA conditions, not a quote — we confirm pricing in writing before any work begins.
Last reviewed: August 2026.
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