Ok I just found out that I have an SES code P0420 which is something like catalyist below threshhold bank 1. Now I did a search for this and found that it means drivers side rear O2 sensor could be bad and to figure that out, you should swap the driver rear with pass rear sensors to see if the problem follows. The only problem is that on the V-6 there is only one cat and 1 rear O2 sensor. I noticed in my seach it was all under the V-8 forums. Now I would have figured that this code ment the forward O2 sensors since those are the only ones that can be bank 1 or bank 2 but under the V-8 sections it specificly said not the forward ones, it is the rear ones. So if my V-6 only has 1 rear O2 is that the one I should change? I am pretty sure its not a cat since I just had a new exhaust and put in a new cat while I was there. It has less than 10K miles on it. So my question is, on the V-6 should i change out the rear O2 sensor or the drivers side (bank 1) forward O2 sensor?

 

My GM Manual for the V6 say's this:

DTC P0420 Catalyst System Low Efficiency

Conditions for setting the DTC P0420:
The PCM determines that the catalyst's oxygen storage capacity is below a threshold considered acceptable

okay, you have a new cat..... but check all conditions which could damage the cat: Misfire, high engine oil or coolant consumption, retarded spark timing or weak spark! This can kill your cat and you see again the P0420.....

Or check your exhaust system for leaks!

Or at least your O2 sensor (the one behind the cat) or its harness is not okay (I think this)

hope this helps a little bit, if you have more question please send me an PN or post it here

 

there's one o2 sensor directly before the cat and one directly after the cat

 

only on certain year models... mine didnt have a pre-cat sensor and now it doesnt have a post-cat sensor