sedan vs auto

sedan and auto both are nouns.

Word	Noun	Adjective	Verb	Adverb
sedan	Yes	No	No	No
auto	Yes	No	No	No

As nouns, auto is a hypernym of sedan; that is, auto is a word with a broader meaning than sedan:

sedan: a car that is closed and that has front and rear seats and two or four doors
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

Other hypernyms of sedan include automobile, car, machine, motorcar.

sedan (noun)	auto (noun)
a car that is closed and that has front and rear seats and two or four doors	a motor vehicle with four wheels; usually propelled by an internal combustion engine
a closed litter for one passenger