NOx Sensor B invalid data. U059F DTC code

[RMADSEN]

I have a 2012, 4500 C&C stock. 67k miles. We just put 13k on it going to Alaska with a Lance camper on top. On the way to Redding,CA from the coast 2 codes popped up. P2509 - ECM/PCM input signal intermittent and U059F - invalid data from NOx sensor B. I did a reset and after 200 miles the U059F code has come back. I did get the recall on the CAT completed in 2018. I see that people have had to pay up to $500 for the sensor. Can someone tell where it is located and if there is a way to test to see if it is good or bad. Can they be purchased for less than $500?