For some time now, we have been getting a sewer odor in our main bathroom that seems to come and go. It always seems to be worse in the Fall of the year. Our main sewer line goes directly from this toilet into the ground (other drains are connected to this main pipe). By the way, this is a single story house. I have been under the house, and there are no visible leaks. I have also re-seated this toilet and installed a new wax ring. Is it possible that the sewer line is leaking air into the house?
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Is it possible that the sewer line is leaking air into the house Not very likely unless a vent opened up inside your wall.
Is there any bubbling when you flush or drain?

we have been getting a sewer odor in our main bathroom that seems to come and go. It always seems to be worse in the Fall of the year. There's a reason for this. Sewer gas raises with warm weather and falls with colder weather. In the fall your windows are open letting in colder air so you smell more of it in colder weather.
If this were my call I'd do a smoke or peperment test on your drainage and vents. That checks for hidden leaks in the wall or open vents in the attic, Sound like a plan? What do you think? Tom

I've been thinking that it might take a smoke test to find the root cause. Is this something that most reputable plumbers can do?