Just replaced ECM and now it will not start. Same serial # as original too.

Reinstall old ECM and troubleshoot the actual cause of your P0773 concern. That's what I would do. An ECM seldom repairs anything. Also, you likely have programming to perform such as ensuring the keys are recognized by the vehicle after ECM replacement. However, again, it is highly unlikely (although indeed possible) that the ECM is the cause of your problem in the first place.