Thanks Rick, I had the wrong DLL file for the XDF I was using, saves without the warning now so I'll try writing the file back to the PCM.

So, new OS written to the PCM (12587603), read the bin and opened into tuner pro, switched the VATS off and saved. . . . . . When I save it has a 'checksum' error but it opens back into tuner pro ok,...

Bench harnes finished, wish me luck :)

Thanks, I already have the 7603 OS file and a fairly generic bin file for it, I just need to do some research now and figure how to write it all to the PCM, I'm still building the motor so have time...

I have a junkyard LQ4 with loom and ECU, but unfortunatley it is OS12597120, so no XDF for it, hence not configurable, is there a suitable 'universal' OS that I can flash to the PCM that will work...

Thanks for having me, I'm older than I care to disclose and I've built dozens of engines over the years but never an LS, I've had two 'normal' vehilces with LS engines and have wanted to swap one...