P0505 NISSAN -

Question

After changing ECM, next step to reprogram ignition keys,
Q? 4 digits Immobiliser Key I should use old one or new one coming with new ECM

Answer 1

You need to get a new code for the new ECM. Using the Nissan scanner the dealer will read a code from the ECM.  With the code from the ECM and the VIN the dealer will generate a new pin code to reprogram the your key to the new ECM.