a guy is selling some ecm on e bay he mention this
IMPORTANT #1: This ECM requires factory programming for your VIN. What does this mean and why is this necessary? This does not just mean a 17-digit VIN is "burned into the ECM". The entire flash memory is erased and rewritten with the correct GM program for your vehicle (AND the VIN is changed to match yours). That's why you can't just install any used ECM with matching service number; it may not work correctly or at all. The problem is not just "the VIN" but the entire program has to exactly match all your vehicle specs -- engine, trans, axle/tires, fuel system, security system, emissions, many other options, etc. The VIN only serves as a pointer to download the entire correct program from GM database to your ECM.