Ok so I just installed my header and catless downpipe. The problem is
There's not a bung for the 2nd o2 sensor. My check engine light is on and
I'm getting 13 mpg. Does anyone know how to fix this?

 

uhhh. get a bung welded in and get an o2 spacer.

 

for one use the search button and use hpt to turn the light off and keep your foot out of it

 

The second o2 sensor only measures cat efficiency, nothing else. You would not see bad gas mileage from it, only from the primary o2. I have had my secondary o2 unplugged for a while, check engine light on, but same efficiency.

 

As have I. My second o2 sensor is completly uninstalled, and the second hole in mine is plugged. I just deal with the CEL, and swap back to stock for inspections, and erase the CEL with the stock DP on there and the o2 place and plugged in.