Does my vent stack wye off of the drain under my slab?

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Apr 28, 2012
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House ~1945, full basement and 2 stories. I am replacing a section of cast iron pipe (long story, already demo'd). The waste pipe descends from the 2nd floor to 1st floor (no connections) to the basement in the far left corner in the photo, which is not the corner of the house. In the photo, there is a wye about 30" from the wall, which runs parallel to the outer wall - see red arrow. There is no other plumbing on that side of the house; no basement floor drain. There is an iron pipe cut off at level grade at the corner of the house ~10' from the picture that used to accept the downspout. Plumbing a downspout to the sanitary sewer was a common practice in this area decades ago. Mine has been appropriately rerouted to drain 8' from the house. There is a full bath in the basement if that makes a difference, it is off to the right.
  1. Is that a wye to a vent stack? I thought that my vent stacks started higher.
  2. If the wye doesn't lead to a vent stack, where does it go?
  3. If the wye isn't a vent stack, what do I do with it? My camera shows that section to be in good shape and I'm tempted to Fernco to the wye on both sides without taking out the cast iron wye or touching the mystery pipe.
Pic below with black arrow indicating location of waste pipe from above in the corner; red arrow is direction of the mystery pipe. The black corrugated pipe is my interior French drain.