Originally Posted by limpy

been there,long story ,short answer,transmission solenoid was sticking,found this after replacing throttle body,gas pedal and hours of checking continuity on related connections between pedal,throttle body and ECM.Nightmare condition!!DTC'S were P1125,P2125 and P2138. I hope this helps,FYI replacing this transmission solenoid is a major job,not one for the average backyard mekanik(which I am proud to be) I found that when I was able to drive normal(my problem was intermittent) I kicked it into passing gear and resumed normal driving,by doing this several times it started to get better,I also would accelerate faster than normal each time I drove my 2006 Chevy Uplander and eventually it kept getting better and finally stayed out of the "DREADED LIMP MODE" I hope this helps you