Replace the exhaust

Bluechic

Hello there,

Sadly, Invidia exhauste on my Z got burn out and have to replace it. I was stop by the Nissan service center and they asked me $4600 to replace it to OEM, I think they also charged on CAT too. I'm planning to buy an exhauste for Invidia again but not sure for cat replacement. I talked with my buddy who has some idea about car, he told me the code PO420 which I got for SES can be occured because of O2 sensor, CAT or exhauste. How should I know what parts do I have to replace for PO420? My exhauste looks burn out (color is almost yellow) but as far as I know, lifespan for cat is quite long. Does anyone can tell me how to make a decision for it? Also, my buddy recommend me to replace O2 sensor in this time if it doesn't expensive.