I was wondering if any of you mechanics out there could help me out. So the catalyst converter for my Toyota Camry was replaced with an aftermarket converter, cuz the original one went rusty and it had a hole on it. Anyway, after a month I'm now getting the check engine light. A friend of mine who has an ODB II code reader checked it out and it reads P0420. So I googled this on the web and their saying that the catalyst is at fault. But how can this be it's a brand new aftermarket catalyst? The other option is to check the oxygen sensors. So i'm not sure what to do, cuz a brand new OEM catalyst is expensive.