p0420 computer code

How can I figure out what specifically this code means? It's lit up on my 2001 Maxima.

Catalyst System Efficiency Below Threshold (Bank 1)
http://autorepair.about.com/cs/troubleshooting/l/bl_obd_main.htm

Catalyst System Efficiency Below Threshold (Bank 1)
http://autorepair.about.com/cs/troubleshooting/l/bl_obd_main.htm

I had this same code, is it something to worry about? The light has since gone off.

I'm not too sure, but it sounds like an o2 sensor to me. you might want to get it changed because if the o2 sensor is bad, your gas mileage will be horrible.

I have the same problem. First time it came up was when I drove up to 7000+ ft above sea level. Then I added a catback and an intake and it was fine until I drove up to that mountain again. I cleared the code, and it came up couple of days after that. I read on the org that there's a TSB for that (i think it's only for 2k-1 tho). Also I read somewhere that if you add a catback, it can come on due to loss of some backpressure.