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I too have had the code 0420 present itself on my 2002 Altima 2.5S (actually in the shop this moring to get that fixed). I'm at about 110,000 trouble free miles... so this is the first issue for my Altima. If your car is still under 80,000 miles... then the catalytic converter is under a mandatory EPA warranty (8 years/80,000 miles). The code 0420 is actually something to the effect that the catalyst is below efficiency. That doesn't mean it's the O2 sensor... but it doesn't rule it out, either.
I would get my car to the shop for proper repair soon. Given that your car is still under 80,000 miles... your catalytic converter will be replaced with no cost to you (assuming that's what is wrong).
Good luck, but from what I understand about this sympton, you don't want it to go unchecked...