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P0420 usually means the catalytic converter has failed and if its clogged it can cause vehicle to run rough, P0300 code, oxygen sensoor codes, and egr codes...You can remove oxygen sensor before converter to see if it runs better or install pressure guage, pressure shouldnt go over a few psi whiel reving engine.
could it be that the O2 sensors are not working properly. Or that I'm using gas that has ethonal in it. I put gas in it the past few days and i'm not having the problem right now.
I think it could just be a O2 sensor that I bought that is not working. I could not find any leaks in the manifold or exhaust system. I replaced the fuel filter last year.