my truck will occasionally turn on the MIL, showing code P0138, 02 sensor voltage high. anyone had this problem before? do I need a new sensor? dumb question but where is the 02 sensor?

I slid under the truck and looked near the cat, where is was on the 4.7 on my old grand cherokee and didn't see it.

 

Should be sensorsnear each manifold and behind the cats.
Should be driver side after the converter. for the P0138. Perhaps the converter is failing if the sensor isn't bad.