I can get the part number from work today, there is an adapter that we install when we do cat back and highflow exhaust on lancers and 4th generation eclipses. What the adapter does is it moved the O2 sensor out further from the pipe so that the element isn't picking up as much air/fuel flow. The part threads into where your sensor goes and then the sensor threads into the adapter.

 

or he could look at our write-up on how to do it. Its only like $5. I think this is what moronicicon is talking about
https://mitsubishiforum.com/m_94383/tm.htm

 

While we are on the subject. I had mine on the lift and noticed there is a plug where the rear O2 sensor would go and there is no wiring in the area that is even close.

Anyone know where the wiring for the rear O2 sensor is fed from/where in comes out of the car.