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Sewer odor detection in San Jose: finding where the smell comes from
A sewer smell that comes and goes is a frustrating thing to live with, because the source is rarely where the smell is strongest. It is usually a dry trap, a failed seal, a cracked vent or a drain leaking inside a wall or under the floor, and each one is found differently.
Sewer odor detection in San Jose starts with the simple causes, dry traps and toilet or drain seals, then moves to vents and hidden drain lines. A smoke test fills the drain system with harmless smoke so it escapes wherever there is a crack or open joint, showing exactly where sewer gas is getting into the house.
The job
Why the smell is hard to trace
Sewer gas is kept out of a house by water sitting in every trap and by sealed pipes and vents. When a trap dries out, a seal fails or a pipe cracks, gas escapes and drifts through wall cavities and under floors before anyone notices it. That is why the smell is often strongest in a room far from its source, and why guessing at the cause wastes time and money.
When the smell traces back to the lateral, the next step is a sewer camera inspection. Cracked old drain and vent pipe is replaced under cast iron drain pipe replacement.
DIY or a plumber
What to check first, and what to leave
A few causes are easy to check yourself before calling. Hidden leaks and vent problems are not.
| Situation | Do it yourself | Call us |
|---|---|---|
| Unused drains | Run water into guest showers and floor drains | A smell that returns within days |
| Toilets | Check for rocking or water at the base | A seal that keeps failing |
| Sink overflows | Clean the overflow opening | Odor coming from inside the wall |
| Washer standpipe | Look for a trap under the laundry box | No trap, or a hidden connection |
| Outside | Make sure cleanout caps are on | Smell near the foundation or vents |
Cost factors
What changes the price of finding the smell
Planning range, not a quote: $140 to $220 per hour for plumbers in Santa Clara County. Source: proplumbersanjose.com, 2026-07-26.
- Cause A dry trap is quick; a hidden crack takes testing.
- Access Attics, walls and crawl spaces take time.
- Smoke test Adds equipment and setup.
- Repair The fix is priced once the source is found.
- Time of call After-hours is billed differently; you hear how first.
Every job is priced after we see it; any published range is for planning only.
Smoke testing
A smoke test pushes harmless, visible smoke into the drain system, usually through a cleanout or the roof vent, with the fixtures sealed. The smoke follows the same paths sewer gas does and appears wherever there is a crack, an open joint or a failed seal: at an outlet, under a cabinet, through a floorboard or out of the ground. That turns a guess into a mark on the wall. We let you and your neighbors know before testing, so no one mistakes the smoke for a fire.
Dry trap and floor drain odor
Every trap holds a few inches of water that blocks sewer gas. In a guest bathroom, a floor drain or a rarely used shower, that water evaporates within weeks and the smell appears as if from nowhere. The fix can be as simple as running water, but a floor drain that dries out often benefits from a trap primer or a trap seal insert. We also look for traps that siphon dry every time a nearby fixture drains, which points to a venting problem.
Cracked vent or stack
Vent pipes carry sewer gas up through the roof and let drains flow freely. A cracked cast iron stack, a vent joint that separated in the attic, or a vent cut during a remodel and never reconnected will release gas inside the walls. We trace the vents from the roof and the attic, test them with smoke, and replace the damaged section. In older homes, a crack found this way is often a sign that more of the same pipe is near the end.
Under-house drain leak odor
In raised-floor homes, a cracked or separated drain line in the crawl space can leak slowly for months, keeping the soil damp and sending odor up through the floor. We inspect the crawl space, run water through each fixture while watching the lines, and camera-check the drains that cannot be seen. Repairs range from a new coupling on a separated joint to replacing a corroded section, and we tell you if the crawl space needs cleaning afterward. The odor usually fades within days of the repair.
What happens after you call
How an odor call runs
- 1
Call
Where and when the smell appears.
- 2
Simple checks
Traps, seals and cleanouts.
- 3
Vents
Roof and attic vents traced.
- 4
Smoke test
Hidden leaks made visible.
- 5
Report
Source shown and explained.
- 6
Repair
Priced and fixed once found.
Local context
Old cast iron and hidden odors
San Jose's 1884 sewer ordinance required house drains of cast iron or ironstone, which is why the oldest properties around downtown still show cast iron drains and clay laterals. After more than a century in some cases, that pipe cracks along its length and at its joints, and a crack in a vent inside a wall is one of the most common hidden sources of sewer odor. In a house with original cast iron, a smoke test often finds in minutes what weeks of guessing did not.
Warning signs
Signs it is more than a dry trap
- Smell after running water
Gas pushed out of a cracked line.
- Odor in one wall
A leaking drain or vent inside it.
- Smell under the house
A broken drain in the crawl space.
- Gurgling drains
A blocked vent pulling traps dry.
- Odor after rain
Water in a cracked line displacing gas.
- Stains or damp spots
A slow drain leak behind the surface.
FAQ
Sewer odor questions
In high concentrations it can be; ventilate and call if the smell is strong.
Traps dry out and refill, and gas moves with air pressure.
It is designed to be harmless and clears quickly.
Yes, through a failed wax ring or seal.
Rising water in a cracked line can push gas out.
Yes, priced once the source is shown.
Check the cleanout caps; a smell near the foundation may be a broken line.
Related work
Other Drains & Sewer jobs we handle
Sewer smell in your San Jose home?
Call any hour; we trace it to the source.
Updated September 2026