Would a leaking exhaust cause a P0430? I looked closely at my cat installation, and on the pass side front, the guy used a clamp and then tried to weld, but couldn't get all the way around. The possible leak is before the cat and after the front O2 sensor...

 

If it is leaking, yes, that could be your problem.

 

I can hear a slight leak from that side, but they swear there isn't a leak... I have replaced the gaskets for the manifold (inspected it) and the downpipe (inspected it too)... Time to call Magnaflow...I don't think the cat is damaged, but they probably wouldn't like to hear about their recommended dealer giving people the run around. Thanks Mike...