An additive gas delivery apparatus, system and method for delivery of additive gas, such as nitric oxide [NO], to a patient use a control computer configured to determine if a monitored parameter indicates that a patient is connected to the additive gas delivery apparatus, determine if the additive gas delivery apparatus is in a passive state in which it does not deliver additive gas, and activate an alarm if the additive gas delivery apparatus is in said passive state when the monitored parameter indicates that a patient is connected to the additive gas deliver apparatus. The alarm serves to notify the user of undesired non-delivery of additive gas, and so serves to improve safety in additive gas delivery systems.