fire alarm vs warning device

fire alarm and warning device both are nouns.

Word	Noun	Adjective	Verb	Adverb
fire alarm	Yes	No	No	No
warning device	Yes	No	No	No

As nouns, warning device is a hypernym of fire alarm; that is, warning device is a word with a broader meaning than fire alarm:

fire alarm: an alarm that is tripped off by fire or smoke
warning device: a device that signals the occurrence of some undesirable event

Other hypernyms of fire alarm include alarm, alarm system.

fire alarm (noun)	warning device (noun)
an alarm that is tripped off by fire or smoke	a device that signals the occurrence of some undesirable event
a shout or bell to warn that fire has broken out