4X4? Any leaks at the rear of the trans now? I've seen guys break the rear housing causing the "E" shift solenoid code. More likely that the 20 way pig tail has a backed out terminal and or poor pin fit. Also if the transmission internal harness has been leaking into the main chassis harness and it was moved around it could have shorted the wires (very very common on the Allison!!!) AND the very best part is that GM nor Allison sells that pig tail, BUT if you get to a wrecking yard cut two connectors out from a 4L60/4L80 and build your own. Same connector but only pinned out for 11/12 rather than the Allison 20 wires it needs.
Did he/she do anything besides replace the flexplate?
Make sure the dip stick is at the top of cold and that after it gets warm is in the middle/top of hot. Allison is a pre-Madona about fluid level.
Report back if none of that is the case, I can get you schematics so you can start testing wires.
