You probably have a broken stack pipe in the wall going up and outside to the roof. You will need to have that pipe replaced so call a plumber. My husband is very handy with pipe and he was able to do it himself.
While a stack pipe could be the issue, this does not happen very often. Odors such as these can be caused by several factors. I would assume that the bowl is holding water on the bottom. So that rules out any odors coming up from the main drain into the bowl area. If the tiny holes that assist the flush around the ring of the bowl are plugged from water hardness that can be a cause.. The rim of most toilets are hollow. When you flush a portion of the water that enters into the bowl chamber goes into that hollow ring where the water is allowed to flow out of the holes to wash the bowl clean. As these holes fill up with hardness from the water, not only does the bowl not clean and flush as well, but some of the water remains inside the ring unable to come out. Over time this water creates bacteria that oftentimes results in all sorts of foul odors. Both from splash backs up when toilet is used and from the water itself.
My suggestion is to use a mirror to look up under the ring around the bowls edge and using tiny picks or something that will not crack the bowl open up these holes. Or assure that they are open. Those that are plugged may be the cause of your issue.
Other then that and the toilet not having water staying in the bottom of the bowl those are the only two reasons that I am aware of that would cause odors from the bowl itself.
1 in a million its the pipe. Wood bridge sounds like they know what they are doing.
Woodbridge is on target...... but My Girl friend found out that here bath Matt was getting wet that was made of rubber and over a period of time the chemical from cleaning it change the rubber and broke it down and it held the odor.......threw out the MATT and has no more Problems........
I have this same problem in a master bath on my second level. I have replaced everything. Caulked around the flooring but still the smell continues. Help?
You may have seepage from the wax ring . This is not real common but may very well be it . If not someone is peeing on the mat tell them to turn light on.
I had the same issue a few years ago. I did this quick fix and it is still working: clean with bleach and water all around the base of the toilet (where it meets the floor) allow it to dry, then put a bead of caulk all around sealing the base of the toilet to the floor. Smooth with a wet finger and allow to dry.
Many overlook the cleaning power of vinegar. I keep a spray bottle in my bath and it will kill mold and bleach out the grout around the floor. The smell dissipates quickly.