We have a problem with a sewer gas smell in downstairs room following use of an upstairs shower. The smell comes through the sheetrock wall and Im speculating a cracked drain pipe behind the wall. A plumber Ive spoken to has suggested he use a smokebomb to locate the leak. He said it would run 4+ hrs for labor while he waits for the leak. Can anyone briefly explain what this entails, and could I do this type of diagnosis myself and avoid the big initial financial hit, before he even starts any actual plumbing work? Thanks much.