Generally, Maintenance deals with non-vital utilities, though there might be a little bit of overlap I suppose, like with the ventilation system or something.

Maybe think of the ship as an apartment, and that's the sort of 'Maintenance' we're talking about here. They're the guys that will come over to assess problems, make very basic repairs and replacements, but they'll need to call in the experts for anything more complicated.

With the life support example, they would probably be able to understand whatever read-outs or dials there are to tell that there is a problem, and maybe even asses what kind of problem it is, but they will need help if the fix isn't a simple one.