Start the Car With a Dead Key Fob Battery

The Fiat 500 (312, 2007–Present) — the modern retro hatch, not the classic — uses a conventional remote key with a turn-key ignition barrel on the steering column for almost every trim. That means a dead fob battery is barely an inconvenience: the flat coin cell only stops the remote lock and unlock buttons. The metal blade and the immobiliser chip inside the key still work, so you unlock the door and start the engine exactly as normal. This guide covers both, plus a note for keyless-equipped versions, and how to fit a fresh cell.

Starting the 500 With a Dead Fob (Turn-Key Cars)

The vast majority of 500s have a key barrel, so a flat fob does not stop you driving:

1. Unlock the driver’s door with the metal blade (steps below) and get in.
2. Insert the same key into the ignition barrel on the steering column.
3. Press the clutch (manual) or brake (Dualogic auto) and turn the key to crank the engine.
4. The engine starts normally — the immobiliser reads the transponder in the key head, which needs no battery.

If your 500 is a rare keyless-go version with a START/STOP button rather than a barrel, hold the dead fob flat against the START button, press the brake and push the button — the antenna behind it reads the chip up close.

Unlocking the Door With the Emergency Blade

With a dead battery the remote buttons stop responding, so use the blade.

1. Slide the latch on the side of the fob and pull out (or fold out) the metal key blade.
2. Go to the driver’s door — the lock barrel is in or just behind the handle.
3. Insert the blade and turn to unlock.
4. This can set off the alarm; turning the ignition on with the recognised key clears it.

The Immobiliser Still Reads the Key

The 500’s immobiliser uses a passive transponder in the key, separate from the remote’s radio. Turning the key (or, on keyless cars, pressing START with the fob on the button) lets an antenna energise that chip and read its code. Because the antenna powers the chip, a totally flat fob never stops the engine starting — it only kills the remote locking.

Replacing the Key Fob Battery

The Fiat 500 remote uses a single CR2032 3V lithium coin cell.

1. Slide the latch and remove the metal blade, which releases the case.
2. Prise the two halves of the fob apart at the seam with a thin coin or flat screwdriver.
3. Lift out the old CR2032 and fit the new one with the “+” side facing the same way.
4. Clip the case shut and refit the blade.
5. Press a button next to the car to confirm remote locking works again.

Frequently Asked Questions

Will a dead fob stop my Fiat 500 starting? On the common turn-key 500, no — it starts as normal and you only lose remote locking. On a keyless-button 500, hold the fob on the START button to start.

Which battery does the 500 key take? A CR2032 3V lithium coin cell.

My 500 cranks but won’t fire after the fob died — why? A flat fob can’t cause that on a turn-key car. Suspect the 12V battery, fuel or the immobiliser rather than the coin cell.

Is the door keyhole hidden on the 500? No — the driver’s door has a visible barrel in or behind the handle.

Can I drive indefinitely on just the blade? Yes. The car is fully usable on the blade; replace the CR2032 only when you want remote central locking back.

If a warning light stayed on after a flat fob or weak 12V battery, you can look up exactly what the code means on our sister site autodtcs.com.