this sensor is attached to my catback... when i took off my downpipe the catback was hanging by it lol...

is it the narrowband o2 sensor?

 

Rear O2 sensor. Entirely for emissions to tell the ECU if the catalytic converter is there and working properly. The computer compares the front and rear O2 sensor readings to make sure there's a difference (the catalytic converter in between did something).

 

Also a nice way to get a CEL light if you remove it.

 

Or if you add a test pipe. try not to ruin it, damn thing cost around $200

 

Add a test pipe and just get a spacer

 

Yep front and back are around 300-400 for the set. I do think there may be some other aftermarket ones that do the job though. Forgot which vendor sells them here on EVOM.