We have a sewer gas smell that we notice in the winter months (Minnesota) and tends to be stronger on the coldest days. We have checked the traps, vents, all the basics. It seems to be at its worst, regardless of the temperature, when we run a lot of water, either laundry or showers or both. When that happens, it can get pretty bad. We are on a septic system.

Earlier today this same question was posted so I'll share my same answer.
"I found a similar problem with a house on a septic system. Turns out the vertical drain went directly into a cast iron main drain in the floor, there was no trap. Without a trap thefumes from the septic tank came back into the house. To fix the problem we made a new vertical drain with PVC and with a trap. We cut off the old drain a few inches above the floor after opening the drywall. The PVC was attached to the old drain with a hub."

Sounds like a code forbidden ''S''trap to me. Traps must be a ''P'' trap where it goes down, back up, and is vented where it goes down again. See what Tom has to say.