P0420 pops up after Header kit install

[martinoledad]

I just had the longtube header kit from stainless works put on today. I thought all things were great until i took it out tonight and noticed it was making a weird sound. sounded like either water or air being pushed out at a rapid speed, couldnt tell if it was just me or if something was wrong like a leak somewhere(place were i had the install done was already closed). I just went to go workout and then got back in the truck and started it up and noticed the engine light was on. got my xcal 3 out and ran DTC check and it popped up P0420 and in the description it said something about the CATS. might this be something about the sound i was hearing or is this because im running the truck stock(custom tunes wont be in till sometime next week hopefully).

Thanks,