fail vs default on

fail and default on both are verbs.

Word	Noun	Adjective	Verb	Adverb
fail	No	No	Yes	No
default on	No	No	Yes	No

As verbs, default on is a hyponym of fail; that is, default on is a word with a more specific, narrower meaning than fail:

Other hyponyms of fail include lose track, strike out, default, choke, muff, miss.

fail (verb)	default on (verb)
stop operating or functioning	fail to pay up
deteriorate
prove insufficient
disappoint, prove undependable to; abandon, forsake
become bankrupt or insolvent; fail financially and close
be unsuccessful
fail to get a passing grade
fail to do something; leave something undone
judge unacceptable
be unable
fall short in what is expected