Which is correct?

a) The air conditioner is operating on Cool mode.
b) The air conditioner is operating in Cool mode.

The difference between the two sentences is the preposition used with 'Cool mode.' Most A/C manuals use the preposition 'in' but to me, 'on' sounds more natural. I've asked a couple friends but I'm getting mixed responses.

I can't find a grammatical reference that defines such cases either... please help !