When using the natural gas appliance regulators (item number 6160788 and 6160785) the vent is for air, not gas. (If gas can be smelt from this, there is a puncture in the diaphragm and must be replaced). The vent allows the diaphragm to move up and down to allow the gas to flow. Through the regulator. If this is blocked, the gas will either be blocked from flowing or will not be able to stop flowing depending on how the diaphragm is stuck. The yellow cap should be left on to prevent dust or insects from blocking the hole.