Why does my Commode smell but no leak?

The sewer gasses are coming back up from a vapor leak somewhere in the waste line. In my experience as an amateur DIY'er it is most likely that the wax ring directly beneath the commode unit has failed and is allowing the gasses to come back up.

I replaced the wax seal yesterday and it made no difference. I even crawled under my home and examined the Plumbing underneath and everything was dry and completely odorless. It really has me stumped, the only thing that i haven't tried is replacing the commode. It is about 20 years old but appears to be in Excellent shape.