Its not the remote code thats the issue its the next step that comes out of the BECM to the ECU to say its ok to start.
I also belive that the ECU sends some sort of acceptance that it matches which then allows the BECM to allow cranking/fuel/spark.
Done a lot of reading and I may have misremembered some of that
Need to check if that signal from the BECM is sent when the fob is used to open or when the key is turned in the ignition.
There is an option to just put it in a working GEMs car and start it, then remove the ECU good to go standalone. Probably have to put the original in learn when replacing. But there is a distinct lack of RRs here

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J