Nissan Repair: 03 lance check eng lite, bench coach, vacuum leak

Question
On my 4 cylinder car it has a code of P0421. Def: Catalyst efficiency low-bank 1. ( o2 sensor monitors the Cat convertor ability to store oxygen) Possible causes:1: Cat Convertor 2: Eng misfire 3: Large vacuum leak 4: oil leaking into piston rings or vlv guides.  The engine has about 90 K miles. It seems to run O.K. the check eng lite has been on intermittently for 12 months. How should I approach this? To save the most bucks.I replaced the spark plugs with no help. Thanks for your help! I don't want to replace the O2 sensor unless I'm sure that is it.

Answer
Hi Lynn -

First of all, the CEL is on for many reasons, most of the issues related to it are benign. I have known many owners who just ignore this light and they are fine.

Anyway, although it is difficult to answer your question accurately, I can tell you that the O2 sensor is the most common component that activates the CEL.

I wish I could be of more assistance, but without the vehicle in front of me, it is difficult to diagnose and be a "bench coach".

Hopw this helps -
aloha
calvin