check connectors  c109/x109, c205/x205.

      the vehicles I had with the p2138 has always been the app sensor or bad connection, and the p2127 has been for the 205 connector with high resistance, but also a bad app some times, I always replaced  this connector. and  they have been very intermittent also just like in your case. but yes I also hate this intermittent codes and in some cases we replace the wrong parts assuming the fix and the time it involves too. check the resistance across each connectors I believe C 205 is to blame although i have seen the app do the same a couple of times . was there any signs of water intrusion on this connectors?  i had also a few gm intermittently loosing one of the 5 volt reference or app1 signal and app2 , when that happens just look at the app data if both 5 volts are there then is a bad connector if not is a bad ecm. if the 2, 5volt signals are coming out of the ecm you can bypass  both signal at the app sensor. in fact i have a vehicle I did that 3 years ago and its still to this day with those 2 wires jumped together running great.