Adele: I too, have a similar problem. However, I am not certain whether my odor is sewer, but there is a distinct odor. What I suspect for my case (still need to verify this) is an improper installation of the shower drain. Even though the P-trap contains water, I suspect that the drian does not have enough drop to drain the line. My shower drain line is linked to my commode. I suspect that when I flush, some of the waste matter goes up the drain to the shower. Not enough to exit the shower drain. Just enough to turn the water a fowl smell. What do the experts think of this theory? Thanks for all your help.