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.