I have a 2003 Honda Accord EX Sedan w/ 2.4 L and its throwing a P0420 code after I just replaced the upstream O2 sensor. Everyone says / thinks that its a catalytic converter problem but I think that its something up with the ignition system since when I'm at a light idling its super rough, like I feel like the car is going to fall apart! I just want to make sure that if the catalytic converter is out that it is indeed out before I dump more money into it. How do you even tell if its going out or is out?!