I purchased new construction Nov. '06. About a month ago an odor was coming from the guest bath, but not all the time. A friend told me about Studor Vent so I went into the attic and found a plumbing pipe shaped like a sheperd's hook with no vent or cap. I am assuming I need something to prevent air from coming back into the house? The master bath doesn't have this problem.

dpthurmond

The Studor vent is a mechanical vent that avoids running a stack out through your roof, which is desirable. The mechanism may have gone bad but the odor would not be limited to the guest bath. The attic would be the worst smell if the Studor is bad. Your P trap may have dried out in the sink if this room is very rarely used. Run some water in the sink and tub/shower, flush the toilet and see if the odor leaves. If it I still present we should also eliminate a bad wax ring on the toilet mounting before you start zeroing in on the Studor.

I purchased new construction Nov. '06. About a month ago an odor was coming from the guest bath, but not all the time. A friend told me about Studor Vent so I went into the attic and found a plumbing pipe shaped like a sheperd's hook with no vent or cap. I am assuming I need something to prevent air from coming back into the house? The master bath doesn't have this problem.

dpthurmond
If there's nothing on the end of that "sheperds crook" and the pipe's open then the builder left you with a illegal "local vent". This pipe MUST be run out the roof or revented back into a exciting roof vent. If, indeed, you have a Studor Vent up there then follow Bals advice and let us know.. Good luck, Tom