I have a first floor shower drain that smells of sewer odor. I clean it out and it's ok for about 4 to 6 weeks. Then the odor returns. This shower is used on a regular basis. The trap remains at a consistent level. The water drains normal without any gurgling. It has 2" pvc pipe connected to a 2" wye which is connected to the 3'' main drain. The other end of the wye has a 2" vent stack connected to it which goes vertically thru to the roof. No other drains in the house exhibit this condition. The house is a 2 story with basement and 6+ years old. Any suggestions to remedy this problem will be greatly appreciated.