I have a Monte SS, 2002 146k miles. I got error code 420. I have replaced front and rear o2 sensors, cat, resonator. I have no clue what other things may cause the error. Any thoughts?

 

What did you replace the Cat with? Unless it is COMPLETELY spec'd to the OEM cat it will throw this code, even though the cat is fine.

 

I honestly dont remember. But i had new cat on for a yr with no issues, so Im guess its not the cat, could be wrong tho

 

..Maybe its smokeing to much pot? lol jking

X2 on what mike said (Chibi)

 

It ran for least a yr, with no problems. SO even if it wasnt to spec wouldnt it have thrown code earlier?

 

It could also be a variance of types of gas....

from one gas station to the next....Do you fill up at the same gas station all the time?

Maybe for test purposes try a higher octane and see if that doesnt fix the issue....

Or replace bank1 O2 sensor.... (Thats the one before the cat)....

It could be that the o2 sensor got contaminated.... via oil, antifreeze, etc....

 

I fill at same station. I do use seafoam additive but not often. I have already changed the o2 sensors. Also after i changed the rear sensor it went away immediately, but after a month now it came back

 

Then possibly try another station....

Maybe that station has changed there gas supplier?

What kind of station is it? and do you buy the lowest grade?...

 

I work at the place that has the station I know the supplier personally. I donot believe its a fuel issue though. I found something about a canister?

 

P0420 is strictly for the Cat not working properly. There is a chance you are running rich and fouling out your cats.

What's your MPG been like? Do you track it all the time?