Start the Car With a Dead Key Fob Battery

Last updated: June 12, 2026

When the smart key on your Mazda CX-5 (Mk2/KF, 2017–Present) stops unlocking the doors and a key-warning symbol appears in the cluster, the coin cell inside the fob has run flat. The KF CX-5 is not stranded by this. A metal emergency key is tucked inside the fob to let you in, and Mazda builds a backup into the push-button start so the engine fires even when the remote has no power at all.

It works because the chip the immobiliser reads does not run off the coin cell. The battery only powers the long-range radio that pops the locks from a distance. Hold the fob in the right place and the car reads it directly.

Start the CX-5 With a Dead Fob

The push-button start on the KF CX-5 can still read the fob at point-blank range. Once you are inside:

1. Sit in the driver’s seat and press the brake pedal fully (depress the clutch on a manual). The starter will not engage otherwise.
2. Hold the fob flat 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 wait for the push-start indicator to light, confirming the car has recognised the key.
4. With the fob still touching the button, press START/STOP. The engine starts.
5. If nothing happens, rotate the fob slightly and try again — and keep the metal parts of other keys clear, as they can stop the signal being received correctly.

This is Mazda’s own listed fallback for a flat key battery on the CX-5; the car has no separate slot to drop the fob into.

Unlock the CX-5 With the Emergency Key

A metal auxiliary key is hidden inside the fob for exactly this situation.

1. On the back of the fob, release the catch (or press the auxiliary-key button) and pull the metal emergency key straight out of its housing.
2. 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.
3. Slide the key in and turn it to unlock the driver’s door, then use the central-locking button inside to free the rest.
4. 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

1. Slide out the emergency key first to expose the case seam.
2. Split the two halves of the fob with a thin coin or a tape-wrapped flat tool.
3. Lift out the old cell and read the number printed on it. Early KF fobs (2017–2019) use a CR2025, while later cars (2020 on) often take a CR2032 — both 3V lithium — so match what is already fitted.
4. Fit the new cell with the + side facing up, 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 cell lasts roughly 2–3 years.

Frequently Asked Questions

Where exactly do I hold the fob on a Mazda CX-5 KF? 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.

Is my CX-5 Mk2 fob a CR2025 or CR2032? It depends on the year — roughly CR2025 for 2017–2019 and CR2032 for 2020 onward. Open the fob and read the number on the old coin cell to be sure.

Where is the hidden door lock on the CX-5? 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, or the metal of another key was interfering. Push the brake all the way down and try again with just the fob touching the button.

Will the car keep running once started? Yes. The engine runs normally once going; just fit a fresh coin cell before you next park somewhere awkward so keyless entry returns.

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