Sewer gas smell indicates a potentially serious health and safety threat. The simplest and more common solution to stop the smell is to pour water into the drains of all your plumbing fixtures to replace any water that has evaporated from your drain traps. Plumbing fixtures that are seldom used can cause a sewer gas odor when the p-traps dry out from water evaporation, especially in the dry winter months. These seldom used drains include shower drains, floor drains and sinks. Add this water to these fixtures every few months if they are seldom used to block the sewer odor.
If this simple water adding method does not stop the odor then further testing is advised. One testing method is a smoke test which indicates a pipe break when visible smoke escapes near the pipe breach. Smoke tests are professionally done and require some knowledge to interpret the signs and to accurately plan where to open the walls.
Another safer testing method is the peppermint test suggested by a plumber. Pour $4-$5 of pure peppermint flavoring down the roof vent stack in your house and begin smelling around the house for the peppermint smell. Where you discover the peppermint smell inside you house indicates a location with something not right in the waste piping. If you smell no peppermint in the house then there is less likelihood of a breach in the waste system inside the house.
Cast iron waste pipes have a typical life span of 50-70 years. However these pipes can fail in 30 years or last for 90 years. If your house is more than 50 years old it is time to plan the replacement of your waste piping system which is usually done at the time of bath and kitchen renovation.
A simpler cause of the sewer odor may be from a blocked vent pipe. This is checked by visually inspecting the vent pipe, usually from the roof for free air passage. If a plumbing vent pipe on the roof is blocked by a critter or bird nest, then the system is compromised. This can result in the water in adjacent traps to be sucked out of the fixtures and allow the sewer gas inside the house.
A caller into my KMOX radio show years ago also discovered that a puzzling and persistent “sewer odor” was actually a faulty breaker in her nearby electrical panel. This fire threat was later discovered when the electrical circuit breaker was replaced and the burnt plastic on the breaker was noticed. The sewer smell was actually the pungent smell of burning plastic and ozone from a smoldering electrical short.