A kitchen sink that drains sluggishly despite having a clean P-trap is typically restricted by a buildup of congealed fats, oils, and grease (FOG) coating the interior walls of the main branch drain line further down in the wall. Over time, these organic substances narrow the pipe diameter, reducing water velocity. Another common culprit is a partially obstructed roof vent stack that creates a vacuum, impeding smooth wastewater movement. Verification: Inspect the branch pipe or check if other kitchen fixtures gurgle during use.
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