P0304 and P0421 Lancer 2006
 

Hello,

My car is a Lancer ES 2006.

I have two code, P0304 and P0421:
* P0304 : Misfire code, I just changed sparkplug 6 months ago, the car drive good. I dont know what is the problem?
* P0421: I have 70 000 miles, I'm not very speed in road, I don't think its catalyst.

I think change a downstream sensor, Can some body know exactely where is the downstream sensor? If you have video, TSB,... I will change it by my self.

Thank you very much.