the kind of action or activity proper to a person, thing, or
institution; the purpose for which something is designed or exists;
This edit is from AndyT's comment. It gives you another noun to use that more closely matches the requested usage of qualifying whether or not something is working as designed. And from the discussion in the comments, I now prefer functionality over function.
1 The quality of being suited to serve a purpose well; practicality.
‘I like the feel and functionality of this bakeware’
1.1 The purpose that something is designed or expected to fulfil.
This edit also stems from the comments and discussion. It provides different parts of speech to use in sentences that are structured for such.
verb (used without object)
- to perform a specified action or activity; work; operate: The computer isn't functioning now. He rarely functions before noon.
- to have or exercise a function; serve: In earlier English the present tense often functioned as a future. This orange crate can
function as a chair.
- of or relating to a function or functions : functional difficulties in the administration.
- capable of operating or functioning : When will the ventilating system be functional again?
- having or serving a utilitarian purpose; capable of serving the purpose for which it was designed
Common usages of these words in everyday speech:
"This phone is ugly, but I love its functionality."
"How well does this computer function?" "It is old, so not well."
"Your brand new Ford breaks down all the time. I drive an '85 Dodge, but at least it's functional."