Hi
I have just read your answer to "empi's" question about sewer odor in the laundry and bathroom.We have a similar situation. The laundry room is directly underneath the bathroom. When we run the washing machine we get a terrible odor in both the laundry room and the bath room. This problem first began after a stubborn clog in the toilet this past fall. It is becoming much more frequent. Your answer about a partial blockage
Makes sense, but where and how do we find the blockage?

Run a video of the line, tell the plumber you want a copy of the video and mark the bad spot on the ground