ecu learn key or key learn ecu

[gulfcountry]

Im after info on ecu replacement . In the bridge terminals 4 and 13 on the obd plug and ignition on for half an hour method of key and ecu syncing , is the ecu learning the key or is the key learning the ecu .
If its the key learning the ecu Id guess you must do this procedure for both keys .

[davo94]

The ECU is learning the key, Yes you will need to do it for all keys. If the key beeper is disconnected the process will fail.

[gulfcountry]

Between the key and the ecu is there a immobiliser module that is also involved in the proces or is the key direcly talking to the ecu and the only changes that are happening are in the actual key fob ?