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.