Just been and looked at mine (there are two) on my fortwo. The forward (at the throttle body) facing one is a doddle to remove and refit though mine doesn't have aircon so easier. The other one's end disappears to god knows where (I have replaced it in the past (by method mentioned)) but as it is fine, I'm digging no further. Both can be checked (for not closing which causes crankcase pressurisation) by detaching them at the throttle body end and blowing into their ends. Your cheeks should swell and your face turn blue if they are closing. Doesn't check that they aren't stuck closed - requires sucking.
I'd check first before replacing either. Replacing either or both is a job any garage can do.