WARNING: This page is about a different car, the 2013 Nissan Armada. However, it is still accessible from the selected car via links, so may be relevant.
DTC No.
Trouble diagnosis name
DTC detecting condition
Possible cause
P0603 0603
ECM power supply circuit
ECM back-up RAM system does not function properly.
Harness or connectors
[ECM power supply (back-up) circuit is open or shorted.]