Hey guys I have a 2000 xj sport.

It has a check engine code of po138 which is rear o2 high voltage. Checked all the wires all look good.

What I need to know is does this sensor actually control my mileage?? I've heard both. And want some clarification thanks!!!!

 

The rear o2 has more to do with emissions. The upstream o2 is the one that has to do with air/fuel fixture, thus affecting your mileage

 

Thanks! I've just heard that If the rear goes bad that it automatically sets the jeep to run rich???

 

Not true about what you've heard about running rich.

A P0138 is most likely a failing oxygen sensor.

Here is a link to more information that code:

http://www.obd-codes.com/p0138

If you plan on keeping the vehicle and these are original sensors, I'd recommend you replace all of them as if one is failing, it's likely that the other(s) are not far behind. Oxygen sensors have an average lifespan (depending on engine efficiency) of 100k - 150k. I use and recommend NTK oxygen sensors, the supplier of OEM sensors to Jeep.

 

X2 on what tjwalker said.

Rockauto.com has the NTK's for $38-$42.

 

Another update the cel keeps turning on/off every other day??? Is that normal of a failing sensor??

 

It's not impossible that you have an intermittent short in the O2 circuit.

But it is way more likely that the sensor itself may have something that is marginal intermittent.

I'd really start with a sensor change here.....They aren't that expensive and it is the first step here.