Front or rear O2 sensor - P0420 CEL

[Catalyst]

I know. Trust me, I've spent that better part of the last 2 hours reading and searching Nabisco and here, just hoping for some more clarification on this.

 

My wifes 2009 Legacy 2.5i has been throwing the infamous P0420 CEL for a little while now and the car is running not great (stumbles at idle and almost stalls at lights). From what I've read it's probably the cat, but to try the O2 sensor first, cheap and easy. From my reading people seem to be going back and forth about whether its the front or rear responsible for this code, if it's even the O2 sensor at all. Seems more people think the rear is the culprit.

 

Also, and this will sound really dumb now. Coming from my GT, I don't know the NA exhaust. Where is the cat that would throw this code? How many cats in the NA, 2? Can't seem to find a diagram anywhere!

 

Thanks for any help, she is really frustrated by this. I persuaded her into getting a suby because I loved mine and she has had problem after problem with it. It's high miles, over 100k, so any warranty work is right out.

[PoManzAWD]

I've seen brand-new vehicles throw a catalyst inefficiency code, then it goes away and never comes back. There's probably one cat per cyl bank on your 2009 2.5i, then one common downstream O2 sensor who's only job is to throw the catalyst inefficiency code (P0420). It has nothing to do with fuel trim or how the engine runs. If that sensor isn't seeing less O2 in the exhaust stream than the upstream 2 sensors, it will set this code.

 

Could be the sensor, or could be one or both of the cats. Good thing is, Federal emissions warranty is like 8yrs/80Kmi for the cats.

 

Has this vehicle been driven with a misfire for any length of time? Excessive fuel loading will kill catalytic converters in short order. Burning coolant will too. Silicone poisoning will also kill them almost instantly (sealants or silicone spray).

 

Joel