DTC P1612: Chain Of ECM-IMMU: Description
WARNING: This page is about a different car, the 2015 Nissan Armada. However, it is still accessible from the selected car via links, so may be relevant.
BCM performs the ID verification with ECM that allows the engine to start. BCM starts the communication with ECM if ignition switch is turned ON and starts the engine if the ID is OK. ECM prevents the engine from starting if the ID is not registered.