What is this part on the cat/downpipe

[tom3]

Did you get the cat back system built in a shop or buy an aftermarket system to install yourself?

 

[Doubeleive]

Cat gutted, also replaced exhaust with a cat back system. No more rattling. Guess we will see if it fails emissions next time I go in.

it will be fine there is no sensor or anything, so as long as there is one there it's a visual pass

 

[fatmonk8]

Did you get the cat back system built in a shop or buy an aftermarket system to install yourself?

Aftermarket and installed myself.