I believe that's for the o2 sensor. Can you see if you have two o2 sensors plugged in? One before the cat and then one after? Looks like a custom cat delete? That may be the plug for the second o2 and it's turned off in the tune so no CEL shows.

 

That looks like the upstream 02 sensor plug, the downstream plug should be blue.

-Steve

 

Got a chance to crawl back under the car tonight. Straight pipe all the way to the muffler at the back. Previous owner didn't mention the cat delete but luckily they don't inspect cars or check emissions here. Thanks for your help!

 

...but luckily they don't inspect cars or check emissions here.

Um... where's "here"? It sounds like a beautiful land of freedom. I hope to visit some day.

 

Do you have an upstream o2 sensor? Car wont run right without one.

-Steve

 

This.

Looks like the cat delete is welded directly to the header. Might want to slap the previous owner, because you'll have to cut that to get the header off.

 

Found the blue plug last night too. Both plugs are just dangling

 

You'll NEED to get the green one at least hoked back up with the correct sensor installed. Is the check engine light on?

-Steve