Just looking at page BL-82 in the workshop manual, it does say that the key needs to be registered into the ECM.
"NVIS (Nissan Vehicle Immobilizer System-NATS) has the following immobilizer functions:
Since only NATS ignition keys, whose ID No. have been registered into the ECM and BCM, allow the
engine to run, operation of a stolen vehicle without a NATS registered key is prevented by NATS.
That is to say, NATS will immobilize the engine if someone tries to start it without the registered key of
NATS."
I have found the registration procedure for a new ECU. Well, I think it is that. Taken from page BL-83:
Performing following procedure can automatically perform re-communication of ECM and BCM or Intelligent
Key unit, but only when the ECM has been replaced with a new one (*1).
*1: New one means a virgin ECM which has never been energized on-board.
(In this step, initialization procedure by CONSULT-II is not necessary)
NOTE:
When registering new Key IDs or replacing the ECM other than brand new, refer to CONSULT-II
Operation Manual NATS.
If multiple keys are attached to the key holder, separate them before work.
Distinguish keys with unregistered key ID from those with registered ID.
1. Install ECM.
2. Using a registered key (*2), turn ignition switch to “ON”.
*2: To perform this step, use the key (except for card plate key) that has been used before performing
ECM replacement.
3. Maintain ignition switch in “ON” position for at least 5 seconds.
4. Turn ignition switch to “OFF”.
5. Start engine.
If engine can be started, procedure is completed.
If engine cannot be started, refer to CONSULT-II Operation Manual NATS and initialize control unit
With that procedure there, is it just a case of installing the new ECU and then using the key I've always had with my car, do the whole ignition on ignition off thing, then it should be registered? Or, do I use the key that was used on the new ECU? Can someone clarify?