The BECM code is always the same and is transmitted to the GEMs ecu provided the BECM has already received a valid fob code or EKA code has been entered.The Gems ECU compares this code to the fixed stored code it has and if they agree sends a signal which illuminates the Engine check light and permits engine start.This code can be changed if a BECM is used which has a different code by invoking the relearn via the Faultmate/Nanocom etc.Ie the BECM code does not change at each start,nor does the security relearn happen automatically at each start.
As a matter of interest the Diesel ECU can be set ,when new, to a Non robust mode ,which means it does not have to receive a code from the BECM to operate.However if it has been set to Robust mode,meaning it does need to receive a correct BECM code,then it cannot be reset to Non robust mode.