No sure but occasionally i smell something like foul like sewer gas, seems to be in the evening. There was a bird down the vent last year. I pour a few buckets of hot soapy water down the vent from above, didn't seem to make any unusually draining noise that you expect if it was blocked. So i figured it is ok. Can you experts review this plumbing photo. The bottom is to the sewer, the next on the left is the toilet, then the sink, then bend to the vent up. There is an access point there i might try to get off? You can see on the left the metal, but it seems glued? How do i get that off, with a hammer? Or pour some draino down the vent pipe? Is this configuration even correct?