1999 Sc2

My sister has a 99 SC2 and she is having problems with it starting. Once started however, it seems to run fine. I've already changed the spark plungs.

The only computer code I get is for the oxygen sensor, but from everything I know, they don't work until warmed up so shouldn't affect starting.

What do I check next? Fuel filter, ignition wires?

Thanks,
Richard

Long crank time? Battery might be going on it. I just replaced my battery a month or 2 ago.