These instructions apply to the Mazda 3 Mk3 (BM/BN) 2013-2018. For other models, please choose your vehicle here.
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When the Advanced Keyless fob on your Mazda 3 (Mk3/BM-BN, 2013–2018) stops popping the locks and a key-shaped warning appears in the cluster, the coin cell inside the fob has gone flat. That does not strand the car. The BM/BN Mazda 3 carries a slim metal emergency key folded into the fob to get you in, and Mazda builds a deliberate backup into the push-button start so the engine fires even with no power left in the remote at all.
The reason it still works is that the part of the fob that talks to the immobiliser does not run off the coin cell. The battery only feeds the long-range radio that unlocks the doors from across a car park. Hold the fob in the right spot and the engine reads it directly.
Start the Mazda 3 With a Dead Fob
The push-button start on the BM/BN Mazda 3 can still read the fob at point-blank range. Once you are sitting inside:
- Sit in the driver’s seat and press the brake pedal fully (depress the clutch on a manual). The starter will not engage otherwise.
- Hold the fob flat so the side carrying the Mazda “M” badge touches the START/STOP button on the dash.
- Keep the fob pressed against the button and wait for the push-start indicator to light, confirming the car has recognised the key.
- With the fob still touching the button, press START/STOP. The engine starts.
- If nothing happens, rotate the fob slightly and try again — and keep other metal keys away, as they can block the read.
This is Mazda’s own listed fallback for a flat key battery on the Mazda 3; there is no separate slot to drop the fob into.
Unlock the Mazda 3 With the Emergency Key
A metal auxiliary key is hidden inside the fob for exactly this situation.
- On the back of the fob, slide the small release catch and pull the metal emergency key straight out of its housing.
- Go to the driver’s door. The lock barrel sits behind a small cap on the door handle — prise the cap off with the key tip to reveal it.
- Slide the key in and turn it to unlock the driver’s door, then use the central-locking button inside to free the rest.
- Climb in and 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 reader. The immobiliser reads the chip’s code, confirms it is your key and releases the engine. The flat coin cell only ever powered the radio that unlocks the doors from a distance, which is why proximity entry stops but starting does not.
Replace the Key Fob Battery
- Slide out the emergency key first — on this fob that frees the case.
- Using the gap the key left, split the two halves of the fob with a thin coin or a plastic pry tool.
- Lift out the old cell. The Mazda 3 Mk3 fob uses a single CR2025 3V lithium coin battery.
- Fit the new cell with the + side facing the same way as the old one, handling it only by the edges.
- 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 3 BM/BN? Press the side with the Mazda badge flat against the START/STOP button on the dashboard. The backup reader coil sits directly behind that button.
Which battery does the Mazda 3 Mk3 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 3? Behind a small cap on the driver’s door handle. Prise the cap off with the emergency key to reach the barrel.
The indicator lit 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 running once started? Yes. The engine runs normally once going; 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.
Disclaimer: The information on this page is provided for general guidance only. Always follow your official service manual and safety precautions when working on your vehicle. We are not responsible for errors, omissions, or any damage resulting from the use of this information.
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