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?