I just got my exhaust done and i put in a K&N Intake in my 2000 Chevy Monte SS. I got a Stainless Steel ZZP downpipe with Cat and two mufflers. Now my service engine light is on, so i went to autozone and scaned it and its saying PO420-The oxygen sensors monitor the cat converters ability to store oxygen, probable causes, Faulty Cat. 2. engine misfire, 3 large vacuum leak, 4 engine oil leakage into exhaust valve,or piston rings.

I got the exhuast done at meinike, did the intake myself and the service engine wasn't on before i brought it to the shop. What do you think the problem is?

 

The ZZP downpipe has a high flow cat - it is emissions legal but it doesn't filter near as much as the stock one so your oxygen sensor is noticing the dirtier exhaust coming through and thinks your cat is failing.

You have 2 options, have your PCM re-flashed and delete that code so it doesn't throw the SES light or try the anti-fouler method.

Here's one thread I found on it

https://montecarloforum.com/forum/he...foulers-29498/

 

I had my PCM re-flashed by PCMforless.com. I had them bump up the performance and had the rear O2 removed entirely. In addition I removed the CAT and had an import pipe made so I could swap it out when I wanted to for inspection purposes.