Well first my car is a 2005 dodge stratus, 2.4L engine, 2 door. The problem i have is that sometimes while its idle it shudders lightly, but the big problem is it shudders when i accelerate before i get to 2nd gear. i have to take my foot of the gas and apply it again for it to go. The check engine light is on and autozone gave me error code P0421.

ive replaced the upstream o2 sensor, cleaned the catalytic converter, and cleaned out the fuel injector, what do you think the problem could be?

 

how did you clean converter ? do you have any exhaust leaks ,check spark plugs a misfiring plug can cause cat error codes if need replacing use champion OEM plugs dodges are picky on plugs ,check o2 downstream sensor use dodge parts have seen many people on here buy aftermarket o2 sensors have problems