Sewer gas is backing up through the drain in one of our showers. We figured it was an issue with improper venting. I just tore out my shower, for other reasons, and planned on fixing the vent.
This is a two story house and the issue is in the master bathroom on the second floor. The toilet and shower each are vented with 1 1/2" pipe, but the vent pipes join together in the attic and run to the main 3" stack. When you flush the toilet, the water in the shower trap bounces a little. On windy days, the water in the toilet and the shower trap bounce. It is these days that the smell is the worst.
It is very puzzling because the smell is not consistent. Our thought is that maybe the toilet only having a 1 1/2" vent is causing an issue with the shower. We are planning on running an additional vent on the shower to hopefully take care of it.
If venting was a problem and not installed right in the beginning, a simple flushing of a toilet can siphon the traps of water seal. They make a simple device called a Studor vent that will correct this problem at each fisture. They are easy to install and work well. Don't confuse this with a automatic plumbing vent! Use a brand name Studor vent. Oatey also came out with a simular product which also would work and it is white. If you can't find this product in your area, email me with your address and I will quote you on the item with shipping. email@example.com
A auto-vent is not, however a studor air admittance device is in many areas. Some areas you can pipe a whole building using these without back venting. In this case of a correction they are allowed as long as access is available.