A severely blocked or sluggish drain typically emits a distinct, unpleasant odor resembling rotting food, decaying sulfur, stagnant swamp water, or wet mildew. This foul scent is produced by anaerobic bacteria breaking down trapped organic matter—such as hairballs, grease sludge, and food particles—inside the P-trap and pipe walls, releasing hydrogen sulfide and fatty acid gases up through the drain opening. Verification: Clean out the P-trap and flush the line with enzymatic cleaners to eliminate the decomposition odor.
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