The simplest way to detect sewer gas
The most reliable way to test for sewer gas is your nose. Sewer gas has a distinctive rotten-egg or sulfur smell that most people recognize when ready. If you notice this odor near drains, in the basement, or around the foundation, sewer gas is likely present. This smell test costs nothing and works in most cases.
For a more systematic check, walk through your home and sniff around all drains—kitchen sink, bathroom sink, shower, toilet, and floor drains if you have them. Pay special attention to areas where you have noticed odors before. Open windows first so you can smell the difference between indoor and outdoor air. The smell should be strongest near the source of the leak.
If you cannot smell anything but suspect sewer gas based on health symptoms (headaches, nausea, or respiratory irritation when near drains), you may have a slow leak that is not yet producing a strong odor. In this case, move to the physical tests described below.
Key Takeaways
- A rotten-egg smell near drains is the primary sign of sewer gas, and your nose is the most practical detection tool.
- Check all drains in your home systematically—kitchen, bathroom, toilet, and floor drains—by sniffing near each one on a calm day.
- Physical tests like the water seal test and smoke test can confirm sewer gas when odor alone is unclear.
- If you find sewer gas, the source is usually a dried-out P-trap, cracked vent pipe, or broken sewer line, and each requires different repairs.
The water seal test for P-traps
A P-trap is the curved pipe under your sink or toilet that holds standing water. This water acts as a seal to block sewer gas from entering your home. If the P-trap dries out—because a drain has not been used in weeks, or because the seal has failed—gas can rise through the drain opening.
To test whether a P-trap is the problem, pour one cup of water down the drain that smells like sewer gas. Wait 30 minutes, then sniff the drain again. If the smell is gone or much weaker, the P-trap was dry and the water you added refilled the seal. This is a temporary fix; you will need to use that drain regularly (at least once a week) to keep the trap full, or the smell will return.
If the smell remains strong after you add water, the problem is not a dry trap. The gas is coming from a crack in the vent pipe, a broken sewer line, or a leak in the trap itself. You will need to move to the next test or call a plumber.
The smoke test to locate cracks and leaks
A smoke test uses non-toxic smoke to trace the path of sewer gas and find where it is entering your home. You can rent a smoke machine from a plumbing supply store or tool rental shop for $30 to $50 per day. The machine produces a harmless white smoke that follows the same path as sewer gas.
To perform a smoke test, close all windows and doors, then have the plumber or rental technician introduce smoke into the sewer line through a cleanout (usually a capped pipe in your basement or yard). Watch where the smoke appears—it will seep out of cracks in the sewer line, broken vent pipes, or gaps around foundation penetrations. Mark these spots so a plumber can inspect them.
Smoke tests are most useful when you have confirmed sewer gas is present but cannot find the source by smell alone. They are also the standard test plumbers use before recommending repairs, so if you hire a professional, they will likely perform this test themselves.
Testing for sewer gas near the foundation and yard
Sewer gas can also leak from the main sewer line where it exits your home and runs to the street or septic system. Walk around the outside of your home and sniff near the foundation, especially in low spots where water pools. A strong rotten-egg smell in the yard, near the septic tank, or along the sewer line route suggests a crack or break in the underground pipe.
Look for other signs of a broken sewer line: patches of unusually green grass (fed by leaking sewage), soft or sunken soil, or cracks in the foundation or driveway. These physical signs combined with a sewer smell indicate a serious problem that requires professional inspection and repair.
If you suspect a break in the main line, do not delay. A broken sewer line allows sewage to contaminate groundwater and soil, and it can damage your foundation. Call a plumber to perform a camera inspection of the line, which costs $300 to $800 but shows exactly where the break is and what repair method will work.
When to call a plumber instead of testing yourself
You can perform the smell test and water seal test on your own with no tools or cost. However, if those tests do not solve the problem, or if you suspect a crack in the vent pipe or main sewer line, hire a plumber. Plumbers have the equipment and training to safely locate the source and recommend the right repair.
Call a plumber when ready if you notice sewer gas combined with slow drains, gurgling sounds from the toilet, or sewage backing up into the shower or tub. These signs indicate a blockage or break that is serious enough to affect drainage, and the problem will worsen without professional repair.
A plumber will typically start with a visual inspection and smell test, then move to a smoke test or camera inspection if needed. The cost of diagnosis is usually $100 to $300, and knowing the source before repair begins saves money on unnecessary work.
Common mistakes when testing for sewer gas
The biggest mistake is assuming any drain smell is sewer gas. Some drains smell bad because of mold, bacteria, or food debris—not sewer gas. True sewer gas has a distinctive rotten-egg or sulfur smell that is hard to miss. If you are unsure, sniff a drain you know is working well (like a kitchen sink you use daily) and compare. A healthy drain should have little to no smell.
Another mistake is testing on a windy day. Wind can carry outdoor odors into your home and make it hard to pinpoint where the smell is coming from. Test on a calm day, and close windows and doors so you can isolate the smell to a specific drain or area.
Do not ignore sewer gas for weeks hoping it will go away on its own. While a dried P-trap is straightforward to fix (just use the drain regularly), other sources like cracked vent pipes or broken sewer lines get worse over time and become more expensive to repair. Test early and call a plumber if the smell persists after you refill a P-trap.
Frequently Asked Questions
Is sewer gas dangerous to breathe?
Sewer gas is not toxic in the small amounts that leak into homes, but prolonged exposure can cause headaches, nausea, and respiratory irritation. If you smell sewer gas regularly, find and fix the source rather than living with it. Open windows to ventilate the area while you investigate.
Can I test for sewer gas with a carbon monoxide detector?
No. Carbon monoxide detectors do not detect sewer gas. Sewer gas is primarily hydrogen sulfide, which has a smell but no color. Your nose is the best tool for detection. If you cannot smell it but suspect it is present, hire a plumber to perform a smoke test or camera inspection.
What if the smell comes and goes?
An on-and-off sewer smell usually means a P-trap is drying out intermittently. This happens when a drain is used infrequently—a guest bathroom, basement sink, or rarely-used shower. Use the drain at least once a week to keep the trap full. If the smell persists even when you use the drain regularly, the problem is a crack or leak elsewhere.
How much does it cost to fix sewer gas once I find the source?
Costs vary widely depending on the source. A dried P-trap costs nothing to fix (just use the drain). A cracked vent pipe may cost $300 to $1,000 to repair. A broken main sewer line can cost $3,000 to $25,000 depending on the location and repair method. Get a written estimate from a plumber before work begins.
Can I use a home test kit to detect sewer gas?
Most home test kits sold online are not reliable for sewer gas detection. Your sense of smell and the water seal test are more accurate and cost nothing. If you need professional-grade testing, hire a plumber to perform a smoke test or camera inspection rather than relying on a kit.