P0421

[addi]

This place seems pretty quiet but thought I'd give it a shot.
Just finished dealing with P0420 on my miata, and replaced a bad cat. The wife drives a 2003 Protege5 (65K miles) and I'm getting a P0421 code on her car now. Usually that would point to a bad cat, but it was replaced by the dealership last november. something tells me that it's not the cause with it being a year old and not having any misfire occurances since then. I did replace the plugs and wires this summer (NGKs). Could it be one of the o2 sensors causing the code?
Thanks...whoever might be out there. ;)

[addi]

For what it's worth...called the local dearlership that replaced the bad cat last year. The service guy told me that if I'm getting a P0421 code, it points to a bad cat and not the sensors, even if I had a new one put on a year ago.