Sorry to make my first post a question. And let me begin with thanks in advance for any info your are able to give me.
My buddy has an 05 with a 5.7 hemi. Truck ran great until he put a magniflow exhaust on it. Did not touch the cat. Right after, it threw an 02 sensor code so the mechanic replaced two 02 sensors. It then began running rough and hesitating on acceleration, no matter if it is warm or cold. It has thrown codes p0132 and p0300. I know the first is 02 sensor and the second is multi cylinder miss fire. Why would doing something to the exhaust cause the misfire or is this probably just a coincidence? Any help is appreciated. Any ideas what should be checked?

 

Did you do a full exhaust from the front to the back or from the cats back? Did he replace the rear O2s or the front?

 

From the cat back, I am not sure which sensors were changed.

 

I can't see the cat-back causing any issues. I have on eon my 08 and never got any codes.
If he used Bosch O2 sensors, I have heard that they do not work well in dome Dodges. Return them and get a pair from the dealer. The front 2, between the cats and the engine, control how the truck runs. The rear two just monitor the cats to make sure they are working.