97 XJ O2 sensor ?
 

From what I understand on the 97 models the front O2 sensor is what helps determine your air/fuel ratio and the back one is just to monitor the cat is this correct? I have a p0138 code for high voltage on sensor 2. Is sensor 2 the one that goes behind the cat and if so will it effect my fuel milage?

That is my understanding. The rear sensor should have no effect.

How do I know? I chased a problem for a YEAR that had all the symptoms of a bad 02 sensor, but replacing it didn't help a bit. That was at the start of the process. I was completely baffled that it didn't cure the problem, and I sweated and worked and read and thought and tried everything I could think of.

Then one day I was discussing it with my oldest son, who does a lot of the actual wrenching for me, since my job keeps me extremely busy.

Turns out he had changed the REAR sensor, behind the cat. I got a new sensor, he put it in the front, and presto! Problem solved.