Start the Car With a Dead Key Fob Battery

Last updated: June 12, 2026

When the Advanced Keyless fob on your Mazda 2 (Mk3/DJ, 2015–2022) stops unlocking the doors and the dash shows a key-warning symbol, the coin cell inside the fob has almost certainly gone flat. The car is not dead, and you do not need a tow or a locksmith. Every keyless Mazda 2 hides a metal emergency key inside the fob to get you in, and Mazda builds in a backup way to start the engine even when the remote has no power at all.

The trick on this car is that the fob’s transponder does not run on the battery. The coin cell only drives the long-range radio that pops the locks from a distance. Hold the fob against the right spot and the engine will still start.

Start the Mazda 2 With a Dead Fob

The push-button start on the DJ Mazda 2 can still read the fob at point-blank range. Once you are inside the car:

1. Sit in the driver’s seat and press the brake pedal fully (or, on a manual, depress the clutch). The engine will not crank otherwise.
2. Hold the fob so the side carrying the Mazda “M” badge touches the START/STOP button on the dash.
3. Keep the fob pressed against the button and watch for the green push-start indicator to come on, confirming the car has recognised the key.
4. With the fob still touching it, press the START/STOP button. The engine starts.
5. If nothing happens, rotate the fob slightly and try again — the read zone behind the button is only a couple of centimetres across.

This is Mazda’s own listed backup for a flat key battery on the Mazda 2; the car has no separate slot to drop the fob into.

Unlock the Mazda 2 With the Emergency Key

A slim metal key is folded inside the fob for exactly this moment.

1. On the back of the fob, slide the small release catch sideways and pull the metal auxiliary key straight out of its housing.
2. Go to the driver’s door. The lock cylinder is hidden behind a small cap on the door handle; on the DJ a slot or the key tip lets you prise the cap off to reveal the barrel.
3. Slide the key in and turn it to unlock the driver’s door. Once inside, use the central-locking button to free the rest.
4. Climb in, then use the touch-to-button method above to start the engine.

Why Touching the Button Works

Inside the fob is a passive transponder chip that needs no battery of its own. When you hold it against the START/STOP button, a short-range coil behind the button energises the chip by induction — the same way a contactless bank card draws power from a card reader. The immobiliser reads the chip’s code directly, confirms it is your key and releases the engine. The flat coin cell only ever powered the radio that unlocks the doors from across a car park, which is why proximity entry stops but starting does not.

Replace the Key Fob Battery

1. Slide out the emergency key first — on this fob that releases the case.
2. Using the gap the key left, split the two halves of the fob with a thin coin or a plastic pry tool.
3. Lift out the old cell. The Mazda 2 Mk3 fob uses a single CR2025 3V lithium coin battery.
4. Fit the new cell with the + side facing the same way as the old one, handling it only by the edges.
5. Clip the case shut, push the metal key back in and test the buttons. A fresh CR2025 lasts roughly 2–3 years.

Frequently Asked Questions

Where exactly do I hold the fob on a Mazda 2 Mk3? Press the side with the Mazda badge flat against the START/STOP button on the dashboard. That is where the backup reader coil sits — not the steering column or a cup holder.

Which battery does the Mazda 2 DJ fob take? A CR2025, a thin 3V lithium coin cell sold in any supermarket or hardware shop. Do not substitute a thicker CR2032; it can stop the case sealing.

Where is the hidden door lock on the Mazda 2? Behind a small cap on the driver’s door handle. Prise the cap off with the emergency key to reach the barrel.

The green light came on but the engine did not crank — why? You almost certainly did not have the brake (or clutch) fully pressed. Push it all the way down and press the button again with the fob still touching it.

Will the car keep working once it is running? Yes. Once started the engine runs normally; just fit a fresh CR2025 before you next park somewhere awkward, so the doors unlock properly again.

If a warning light stayed on after all this, you can look up what the code means on our sister site autodtcs.com.