I know this question has been asked multiple times but I haven't found anything specific to our situation. So, I figure I'll try asking too.

We have a 2 story house, full basement with 2 separate "sewage lines" running out to our septic system, one on each end of the house. Both tanks were cleaned last summer so that's not the issue. The sewage smell only comes from the sewage line located on the Southern side of the house. It is horrible in the utility room where the line enters the house. Also in this room is our sump pump. From the main line that enters the house, there are multiple things that I see connected to it. Kitchen drain, laundry drain, half bath drains and then the full bath drains. Aside from seeing a bunch of black pvc pipe I really don't know what I'm looking at... sorry.

I closed the air vent in the full bath, shut the door and sealed it off from any air getting it overnight. In the morning, no smell. Opened the air vent for the next night, smell was back the next morning. Assuming the sewage smell was traveling up from the utility room into that bathroom via empty space around the air vent/ducting (the duct work in our house isn't a pretty sight... that's another topic/post.)

Therefore, I pretty much have to assume that whatever is letting the sewage gas leak is down in that utility room. I just have no clue what it could be.

Is there anything that I can troubleshoot myself prior to having a plumber come out?

It sounds like a trap problem or lack of a trap. Do you have a floor drain? If so pour a guart or gallon of water down it to make sure the trap if present is filled with water to prevent air from backflowing through the sewer line into the house. Every drain leading to the sewer should have a trap with water in the p-trap to prevent air flow coming back.

There is no floor drain in the basement. The sewer lines come into our house near the top 1/3 of the basement wall and there is no p-trap. The line coming in is the normal piece you see (sorry I don't know what it's called) where it comes straight in and has a cap on the end. Shooting off to the side of that pipe is the line where it continues to the rest of the house. That line is at an angle of probably 15 degrees, give or take 5 degrees. The house was built in the 70's so maybe that's what the builder did to avoid using a p-trap??

We've lived here for 6 years and this is the 2nd time we've smelled this odor. The last time was this past year and the main floor toilet needed a new o-ring. That time the smell was worse in that specific bathroom rather than the basement. We live in a clay soil area and have had a decent amount of rain lately. Could that have anything to do with it?

Interestingly, the other sewer line entering the house is the same piece that I described above. This one was installed in '95 and in a better fashion I'd imagine. The piece that has the cap on the end is angled down causing the other side to be directed straight up continuing to the rest of that side of the house. That piping is straight lines. Assuming they did it that way to avoid the p-trap as well?

So, with that extra information, what do you think is the next step? Bleach down the shower drain and/or flushed down the toilets overnight?