Plumbing Questions? Ask a Plumbing Expert.
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Do you have an AAV (see attached picture, it could be black instead of white) under the sink? do you smell the odor all the time or only when you use the sink?
Ok, no AAV. So the most likely culprit is a broken plumbing vent pipe probably in the wall behind the sink if the odor is strongest under the sink. This assumes you have no leak at the wall under the sink