It must be plugged up. try to snake it out.
Water Jetting
A vent pipe is located afte a fixture trap to remove dangerous fumes from the system and exhauts them through the roof terminal as the fresh air enters through the fresh air inlet
Return air vents.........
Portal 2 Demo Part 5 - "Pneumatic Diversity Vent"
Installing Soffit Vents
Fresh Air Supply Vent
Airflow Breeze Vent Review