P0420/p0430

Jhill

I'm not sure if your asking what causes a p0420 as in what causes the code or what cause the cat to go bad causing the p0420. From my experience (12 years as a diag/ drivability tech with different car lines) a p0420 code is almost always a failed cat, I can't think of anytime it has not been, that's not the same case for a diesel but on a gas engine it's almost guaranteed (I say almost for the 1 out of 1 million it isn't).

Of your stuff above
Muffler- no as that is after the o2 sensor
Manifold crack- you will have lean code (even though it will actually be rich)
Exhaust leaks- again after the o2 so no
Misfire- obviously have misfire codes
Oil- never seen this even on oil burners and most likely would be damaging the cat so once again its bad cat
O2 wiring or conn- going to have o2 codes (inactivity)
Injector- rich code
Fuel press- rich code

I have really never seen a p0420 and it not be a cat (for gas engines), the manufactures basically test all other sensors first and veryify they are functioning correctly before they will run a cat test and throw a p0420.