If your bathroom sink refuses to drain even though the visible P-trap and tailpiece pipes underneath appear completely clean and open, the blockage is located further down in the main branch drain line inside the wall or the roof vent stack is obstructed. When debris accumulates in the main wall pipe downstream of the trap, wastewater cannot escape. Alternatively, a blocked roof vent creates an air vacuum that halts drainage. Verification: Check if other plumbing fixtures in the home drain slowly or gurgle when the sink fills.
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