We recently purchased a co-op in a small building (4 Units total). We replaced the sink faucet in one of the bathrooms about a month ago. About a week or two ago we started smelling a bad odor in the bathroom. We assumed it was coming from the toilet since we had a similar problem in a previous apartment. We had a plumber come in and decided to replace the bowl, as there was some other problems with it. The bathroom still smells. My husband thought that maybe the wax seal wasn't installed properly when the new bowl was installed so we re-installed it. It seems the smell may be coming from the sink but when I called the plumber again he said it was unlikely and thought maybe the smell was coming from our downstairs neighbor via the exhaust vent. Does this seem feasible? If not what else can it be? I'm at a loss and cannot bear the odor anymore!