Well I've fixed nearly everything on my baby these past few days, after i replaced some relays i had my car checked for codes and im getting the PO420... I replaced my cat in may of last year so im fairly certain thats not the problem. I'm thinking its either a dirty or dead sensor in the back. What would be best for cleaning it? I was thinking of using some B12 or some carb cleaner but i dont wanna cause any damage and id like to avoid shelling out any more money.

 

Did you replace the Cat with an aftermarket one, or an OEM one?

Basically every Aftermarket cat will throw that code eventually. They just don't perform to the strict settings of the OEM.

 

this^

is it because they flow more tho or what?

 

I replaced it with the zzp 2.5" with highfow cat, and i think my computer was just bein silly. I unplugged the battery for about an hour to get rid of the code is was still throwin for my radiator fans, now theres no CEL. Im hoping it was a fluke but im expecting it to pop back up as i drive more.... I'm still beyond pleased that theres no CEL at the moment. I haven't seen my dash so empty in a long time... definately a great day considering my gf is about to get back from her trip too

 

As for P0420, I've seen a bad rear O2 sensor cause this. To my knowledge, there is no "cleaning" an O2 sensor, it needs replaced.
BUT, as mentioned, the after market cats (not just on GM cars) can cause this issue. Now you switched to a 2.5" ZZP highflow cat. I know they warn about this error on the product page. Knowing you have a high flow cat, I'd skip replacing the rear O2 (as it has not affect on performance). From what I see, you have 2 solutions:
1. - Look up the spark plug anti-fouler trick. The idea is to space the O2 sensor out of the main exhaust stream. It then gives the PCM acceptable readings.
2. - Have the PCM tuned and that code deleted.
Yes, some companies sell an O2 emulator. I've read they are hit or miss and only work on certain years. Option #1, less then $10 DIY project. I was going to do option #1 on my Monte after my High Flow DP if I got the 420 error (at the time, I was not sure I'd go for a PCM tune). Luckily, I never had the code pop BUT I since had the PCM re-tuned and that code deleted.

 

Yes, the error code pops up because they flow better, and since it is out of the range when the o2 sensor reports to the PCM, it comes back as faulty.

 

When I put headers on my monte I used that trick ..Because my cat is on the floor in the garage.. No check engine light or codes ..
1. - Look up the spark plug anti-fouler trick. The idea is to space the O2 sensor out of the main exhaust stream. It then gives the PCM acceptable readings

 

gotcha, that's what i thought but wasn't sure, thanks!