Ceiling fixture with odd set of wires

hedbl00

I live in a house that is 3 years old. I just added a light fixture. Upon opening the the prebuilt ceiling mount, there were 2 sets of wires. The normal Black and white but another wire (yellow). There are two conduits. One conduit has white and black and the other has white, black, and yellow. The light either turns on all the time or not at all. Some help would be greatly appreciated. Thanks!

Speedy Petey

Is this actual conduit? Are you in Chicago?

If it is then I would assume the yellow is the switch leg. Have you tried using the whites and the lone yellow to the fixture? White to whites, yellow to black.

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