What cat did you replace with? Some aftermarket cats will still throw the P0420 because the flow is a little greater than stock.

 

As Chibi said, P0420 with a non-OEM cat is common. Not just with these cars, but MANY manufacturers have this issue. It is commonly a result of how tight the EPA is squeezing auto-makers to meet various emissions criteria.

I would stick with the OEM EGR that is NOT causing the P0401. To my knowledge (and I am allowed to be wrong), the EGR and the P0420 codes are NOT related. You options are:
- Follow the GM diagnostic data and validate there is not a true problem elsewhere in the system (P0420 can be a bad wire, bad cat, bad O2 and some other items).
- Space the O2 sensor out of the exhaust stream. This is a common solution across many manufacturers of cars.
- Pay someone to delete the P0420 code from the PCM to never see it again.

 

Ive supposedly had 3 PCMs that had this code deleted each several times...... ALWAYS comes back.

Is there a reason for this?? Ive had people straight up tell me that cant be true but..

 

Best I can say, it was either not done or done wrong. I have a DHP PowrTuner and I've successfully removed codes.

 

Probably just tuned wrong. They are probably turning off the code, but not telling the PCM to never report it.

 

Ok and something else here... Could a PCM "lose" the timing advance it was supposed to have added and therefore revert to stock timing?? Am I wording that correctly??

 

No, however there are modifiers and other parameters that can alter the timing table. The timing table can't revert itself back, but knock can pull timing, higher intake temperatures can pull timing, etc.

Also if the high octane and low octane tables have different timing, your car could knock and revert to the low octane table. Normally though I just copy the high octane table into the low table, and then subtract a little timing just in case I need to use that table for whatever reason in the Monte.

 

Oh yea man.. Something Ive been told by you guys a few times (obviously not exactly word for word but..).. Dont think your advice goes unheard..

Thank you tho.. As you may or may not know, I have this on going issue with my car and its driving me nuts....