The verb to function has established itself, and is recognised by the dictionaries. But it is something of a newcomer, and there are still those who do not like it. For this reason some other word — work, for instance, or operate, or perform — should be preferred when it will do as well.
Function as a verb, say Whitten and Whitaker, in the sense of act or proceed with is shoddy English, The sentence "The Council declined to function", i.e. to act, is an example. Used of machinery ("the engine refused to function") the verb is admissible.