[Miguelgomez]

i got the P0420 code 2 days ago after installing catted UEL Headers a month and a half ago. I know that usually means either the o2 sensor or the catalytic converter but the thing is ever since the check engine light came on the down shifts make no noise what so ever and they used to be VERY loud and the revs dont go up like before when down shifting. also when the car drops back down to 2nd or 1st gear either in auto or manual mode it shifts very hard. Any idea as to why that is?

Thanks