Theory Of Operation
WARNING: This page is about a different car, the 2015 Fiat 500. However, it is still accessible from the selected car via links, so may be relevant.
The three door lock relays (lock, driver door unlock and passenger door unlock) are internal to the Body Control Module. If one relay is stuck, the central locking system is inoperative. If one is stuck and cannot be freed, the BCM would have to be replaced.