returning truck to stock - mainly ecu?

[Pulse_GTO]

If the engine is in a 2008, and it originally came with a 5.3/4L65E combo, the dealer may be able to flash the ECM and TCM back to factory specifications. Also you may want to make sure that the transmission wiring is correct for when you go back to the 4L65E. The 2008 Tahoe came with the E38 ECM and a separate T42 TCM in the transmission. The T42 was used on 4-speed transmissions and the T43 was used on 6 speed transmissions.

If you have an indipendent shop flash it back to stock using aftermarket software (i.e. HP Tuners or EFI Live), make sure that they first download the file that is currently in your ECM and change that back to stock by comparing values to a known stock file and copying them over to the downloaded file, save the file with a new name and upload the changes. Simply writing over a file from a different vehicle could cause the ECM to brick, VIN to be incorrect and the immobilizer to not allow the vehicle to start.

Do you know if it was even tuned after the swap? Again an independent shop can download your current tune and compare it to a know stock tune and see what changes, if any, have been made. If none have been made, then the shop or individual can copy the parameters over from a stock 6.0 file and it probably will run much better.