What does this code mean?

Can anyone tell me what code "po420" translates to? Just in case it's necessary, this is a 1996 Bonneville; 3800, green, etc.....

Catalyst system low efficiency

Possibilities are cat converter and check rear o2 sensor.

My buddy already changed the rear o2 sensor, so the only other real option is the cat itself?

Did your friend have the code erased with a scan tool after changing the o2?

Well that's a great question I forgot to originally ask. But I just called him and he said he had 3 different codes before; he believes they were PO141, PO300, and PO341. Then he changed the rear O2 sensor and YES hes says he DID erase those codes with a scan tool afterwards. And now this code pops up.