diagnosis vs diagnosing

diagnosis and diagnosing both are nouns.

Word	Noun	Adjective	Verb	Adverb
diagnosis	Yes	No	No	No
diagnosing	Yes	No	No	No

As nouns, diagnosis and diagnosing are synonyms defined as:

diagnosis and diagnosing: identifying the nature or cause of some phenomenon

diagnosis (noun)	diagnosing (noun)
identifying the nature or cause of some phenomenon	identifying the nature or cause of some phenomenon