A few weeks ago I got a check engine light, pulled the codes, P0171 and P0174 (if I remember correctly) the were upstream, O2 sensors, Bank 1 & Bank 2. I figured both O2 sensors don't go bad at the same time so I changed my PCV valve, cleaned my MAF sensor and my Throttle Body, also changed the hose that runs from the bottom of the throttle body to evap system. I had disconnected my battery while working on truck so it reset the CEL.
Drove around for a few hours and the CEL didn't come back on. A day later I got CEL. This time P0136. Downstream O2 sensor. From what I've read, that it caused by low voltage to the downstream O2. But that seems a bit odd a couple days after fixing the upstream O2. Maybe the throttle body cleaner? Maybe I disconnected a wire somewhere near the throttle body or MAF?

 

One really weird thing ive noticed about my 4.0 is that it has given me all kind of codes like these and all from a vacuum leak. I had every part on my intake side leak and I got diff codes everytime lol. I got the oxygen sensor code as well when I deleted my egr but went away the next day lol