Check engine light
 

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

The car thinks there is an issue and is pulling power. Need to get the cel cleared. Do you have a tune?

I didn't think of that. No I haven't gotten it tuned yet. I was going to wait until I finished the full exhaust so I wouldn't have to tune twice but I guess it would be better to tune it now to avoid this again.

You just need to tune for the header anyway. The rest of the exhaust has little need for a tune.

Thanks for the help. Hopefully it goes back to normal performance and sound after

Yep if you tune and clear the code it should go back to normal.