Why is my FIAT 500 key fob not working?

There are a few reasons the key FOB for Your FIAT 500 may stop working. The first and most probable is going to be that the small "watch" battery in it has expired. This'll often be a 3.3 volt lithium, CR2032, which You can pick up just about anywhere for under $3. You'll also find that the remote entry key won't work if the battery on Your vehicle has become too depleted. The solution here, of course, is to recharge it or replace it. Very rarely, the FOB can become desync'd with Your FIAT. In this case, You'll need to either go to the dealer to have it reprogRammed, or follow the procedure in Your manual to do so. Testing can be accomplished by using another FOB and verifying the issue is with the key and not the car.