Intermittent sewer smell in bathroom

We have noticed a sewer smell in one of our upstairs bathrooms that comes and goes. The first time it occurred I changed the wax seal on the toilet (after reading several posts in this forum) and I thought the problem was fixed. Unfortunately, that didn't last. So far, the smell has returned 3 times in the past 2-1/2 weeks. The last time it happened, my wife was taking a shower when the smell went from faint to unbearable. I looked down all of the drains and I could see water in all of them. After reading several posts, I guess it could be a blocked vent pipe, except that the other upstairs bathroom shares the same vent pipe and has never had the smell problem. Both bathrooms are used on a daily basis. One thing I have noticed is that the smell seems to return within 1 day of extremely cold temperatures (lows at or below zero); and the smell dissipates once the cold breaks. Could the extreme cold temperature be causing this somehow?
Any help on this problem is greatly appreciated. I would much rather try to figure out the cause on my own and call a professional only if necessary.
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shad wrote:

Running the shower often seems to aggravate this kind of problem. I'm guessing the hot water flowing down the stack generates more odor. Anyway, don't rule out the wax seal again. Extreme cold temps causing shrinkage to loosen the wax seal? Dunno.
Could you live without that toilet for a day or 2? Take it up and tightly seal the flange opening. See whether there is any odor.
Jim
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How big is the vent pipe. Different areas of the country seem to have different sizes. If that pipe frosts closed in a real cold snap you have a blocked vent pipe. When it warms up the problem goes away.
Just my .02
Ken
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