Fiat 500 Key Fob Replacement
If your 500 key fob doesn't work Try a different one to see if your vehicle can detect it. If it does, the problem is probably with the battery or water damage to the original fob.
Replace the battery first. Make sure that the new # CR2032 coin cell battery is properly inserted, with the "+" side facing down in the fob.
Dead Battery
The majority of the time, when a fob isn't working it's because the battery is dying. This isn't a problem to fix and the majority of fobs will give you a warning when the battery is depleted. You can find the correct procedure for most car fobs YouTube or in your owner's guide. You can also take it to an area locksmith or the dealer that sold it to you to replace it.
Other times, a device can not function and there could be a problem with the circuit board in the internal circuit. This isn't a lot of common but it is possible to happen. If you dropped the fob on something sharp, it could cause damage to the internal chip. It can also happen if you use an aftermarket key and the fob does not function. In this instance you'll be required to go to an authorized locksmith or dealer and have it reprogrammed to work with your 500.

One 2012 FIAT 500 Flip Key Remote - resembles Ilco brand with trunk, lock and lock buttons - new replacement keyless entry remote (will require cutting and programmed by a locksmith)
Chips damaged
A fiat 500 key fob is a wireless device that provides the signal to the car to start when you push the button. It has a microchip that contains the information required to communicate with the car. If the chip is damaged, it will not transmit the correct code and the car will not start. A locksmith can repair this kind of damage. United Locksmith is capable of providing this service for old models as well as newer ones, so don't feel in a rush to call them.
You can purchase a new fob and have it professionally programmed by a locksmith. It's less expensive to do this rather than going to a dealer and it is just as effective. Locksmiths will require specific details from you, like the year your car was manufactured and the model's name.
Rub the chip with an old dollar bill if you're not certain what to do. The copper in the bill is abrasive, and can remove oxidation and dirt. However, it will not harm the microchip. The chip might not be repaired in the event that it has been physically damaged. This could happen if you drop it or hit it by your vehicle.
Water Damage
Whether you accidentally dropped your keys in the water, put them in your pocket while you were swimming at the beach, or splashed them with water on a wet day, it's likely that your fob may have water damage. Even if it only has a tiny amount of water on the key, it could cause serious problems to its electronics and stop it from functioning effectively.
The good news is that you can solve this problem by removing the key fob and using an old towel to scrub it of any dirt or water that remains. After the fob is dry it is possible to replace the battery and it will start working again.
You can also take it to an auto repair shop or locksmith to have it professionally cleaned and programmed. The professionals are able to utilize special equipment to ensure that the key fob is compatible with your car's computer system.
If the key fob on your Fiat 500 stops working, you might feel angry and anxious. It is tempting to head to the dealer to get your key fob replaced however this could be expensive and difficult to arrange. Locksmiths can handle all aspect of your car's key fob replacement without the need for a dealership visit. This is why they are a much better option than an auto dealer.
Pairing Issues
If you accidentally drop your keyfob or it ceases to function and nothing else appears to be wrong, it may be the chip. This could be due to water damage or a dead battery.
Often, this can be accomplished by following the instructions in your owner's manual. However it's recommended to have an automotive locksmith do this if you're having difficulties or the steps in your manual don't work for you. This ensures the chip isn't damaged and is likely to be the least expensive option.
Another issue facing 500 owners is that the key fob will not secure the car. This could be a problem when you intend to fill up at gas stations. Some owners have reported that their 500's locking system has failed and they've had to replace the entire intermediate steering shaft. This issue was cited in a technical service bulletin (TSB) issued by Fiat.
A reputable locksmith can assist you when your Fiat is having problems. They'll duplicate your existing key or create a new one that will lock and open the vehicle. They can do this without seeing your old key, and it's typically cheaper than buying one from a dealer.