malfunction vs break down

malfunction is a noun but break down is not a noun.

Word	Noun	Adjective	Verb	Adverb
malfunction	Yes	No	Yes	No
break down	No	No	Yes	No

As verbs, break down is a hypernym of malfunction; that is, break down is a word with a broader meaning than malfunction:

Other hypernyms of malfunction include break, conk out, die, fail, give out, give way, go, go bad.

malfunction (noun)	break down (noun)
a failure to function normally

malfunction (verb)	break down (verb)
fail to function or function improperly	collapse due to fatigue, an illness, or a sudden attack
	stop operating or functioning
	separate (substances) into constituent elements or parts
	make a mathematical, chemical, or grammatical analysis of; break down into components or essential features
	make ineffective
	cause to fall or collapse
	lose control of one's emotions
	fall apart