Start the Car With a Dead Key Fob Battery

Last updated: June 12, 2026

The Abarth 124 Spider (2016–2020) is unusual in the Fiat/Abarth family: built alongside the Mazda MX-5 (ND) in Hiroshima, it uses Mazda’s keyless system rather than the Fiat turn-key setup. That means a push-button START/STOP ignition and a proximity smart fob. When the fob battery dies, the engine still starts — the immobiliser chip is passive and needs no battery — you just have to present the fob to the button correctly. This guide covers the backup start, unlocking with the emergency blade, and fitting a fresh cell.

Starting the 124 Spider With a Dead Fob

The owner’s handbook gives an explicit procedure for a discharged fob: hold the fob against the START button.

1. Get in using the emergency blade (next section) and sit in the driver’s seat with the door closed.
2. Press the brake pedal (auto) or clutch (manual) fully and hold it.
3. Touch the back of the fob against the START/STOP button, with the lock-switch side facing up and the Fiat badge towards the button. Angle the far end down and use the fob to push the button in.
4. The engine starts. Keep the fob clear of other keys, coins or contactless cards, which can block the signal.

Unlocking the Door With the Emergency Blade

With a flat fob the proximity unlock and remote buttons stop working, so use the hidden metal blade.

1. Slide the release catch on the fob and pull out the emergency key blade.
2. The driver’s door lock barrel is in or just behind the handle.
3. Insert the blade and turn to unlock.
4. This can set off the alarm; it stops once you start the car with the recognised fob.
5. Slide the blade back into the fob before driving.

Why the Engine Still Starts

The 124 Spider fob holds a battery-powered radio for hands-free entry and a separate battery-free transponder for the immobiliser. Touching the fob to the START button lets the antenna behind it energise the chip and read its code directly. So a completely dead fob still starts the engine; only the proximity and remote functions drop out until you fit a new cell.

Replacing the Key Fob Battery

Unlike most Fiat/Abarth keys, the 124 Spider fob uses a CR2025 3V lithium coin cell (the Mazda spec), not a CR2032.

1. Slide the catch and remove the emergency blade, which releases the cover.
2. Insert a small flat screwdriver into the slot and gently part the case halves.
3. Lift out the old CR2025 and fit the new one with the “+” side facing as marked.
4. Clip the case shut and refit the blade.
5. Press a button next to the car to confirm remote and proximity functions are back.

Frequently Asked Questions

How do I start a 124 Spider with a dead fob? Hold the back of the fob against the START/STOP button, press the brake or clutch, and push the button in with the fob. The handbook confirms this exact method.

Which battery does the 124 Spider fob use? A CR2025 3V lithium coin cell — the Mazda MX-5 spec, not the CR2032 used by other Fiats.

Can I use a Mazda MX-5 key on my 124 Spider? The cars share the keyless system, but a fob must still be programmed to your specific car; do not assume an MX-5 fob will simply work.

Can I start the car with just the blade? No. The blade only unlocks the door; the fob must be present at the button for the immobiliser to allow starting.

Why won’t it read the fob at the button? Keep the fob clear of other key fobs, coins, phones and contactless cards — metal and other transponders can stop the antenna reading the chip.

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