PCM and key issue

My kid crossed her jumper cables and fried the main fuse. I replaced the battery and 100 amp fuse. Obd tester gave me a u0100 code. I assumed the original ECM was fried and had a new ECU ordered from the Mazda dealership and replaced the old one (the part number on the new ECU is different than the original, they said it is fine and matched to my VIN number which is what matters, that seemed odd to me). The dealership cut a key for me and when purchased with the ECM told me the key would be programmed for me (lies, after I went to pick it up they told me I would have to tow it there and their tech would diagnose it and then program the key; I walked out). I put the new ECU in and the new key and old key (only one old key) obviously does not start it, but can turn the ignition so my old reader can read any other codes. I still get a u0100 code.
I have looked at the harnesses and connectors, and do not see any corrosion. Any ideas on what to do or why I am getting the same code again? Thanks