The battery in the main key is dead? Unable to deactivate the immobiliser. The spare key is fine because you seldom use it.

tThe immobiliser transponder does not need a beetery and is separate to the remopte control. It may be the immobiliser has lost track of the main key and needs re-associating to the car. I do not know how to do this on Fiats, Either goto the Fiat forum or ask a dealer. I could of course be a responser that has failed but I doubt it. Regards Peter