I have isolated the source of sewer gas to the tub drain in our lower bathroom. I suspect it is a blocked vent pipe. I am not sure what to do next. It is the lower bath in a 3 story house, but still above ground. The vent pipe is not blocked on the roof but I am not sure how to check for blockage lower in the pipe. There is no evidence of leakage around the tub or anywhere else I can see. There are 3 other baths in the house. All the sinks and toilets drain an flush without problems.
I have been in the 32 year old house over a year and the odor started about a month ago. I've removed the overflow and plug and flushed with water, drain cleaner, bleach, and other "miracle" products.
The laundry drain is about 6 feet away but no odor emanates from it or sink or toilet when I seal off the toilet.
Any thought on what to do next short of tearing out the tub.
Thanks in advance.