Okay so I have a 2005 Hyundai Elantra GT with California emissions, and ever since I've had it, it's had p0420 code. And it never really impacted performance, but recently it feels gutless and I know for a fact it's running lean because I pulled the plugs and they were white. I've replaced plugs and wires and the O2 sensor before the CAT(the one on the manifold) and I cleared the code and it hasn't came back on yet. But here's what I noticed when I pulled apart the exhaust: the first manifold CAT looks like it melted, and the second CAT is just completely gone. I'm sure the gutless feeling is from how lean its running. And I have a feeling it has something to do with the CATs and the O2 sensors. But I'm not real sure on how it all works together. Could somebody pleeeeaaaasssee tell me why my car is running lean? And what I need to do to fix it?