Replacing the car battery will not cure this problem. It needs to be plugged into a fiat diagnostic system to be reset. A dealer can do this or an independent technician with the correct software. You could try replacing the battery in the keyfob. You only have 40 seconds to get old battery out a new battery in. Have both battery's on hand. Open up keyfob by pulling off the back you will now see the battery. Slide out old batttery and slide in the new one straight away. Replace cover. Press unlock button for 10 seconds. And test if this has worked? If not the immoboliser is the fault.