These instructions apply to the Mazda 3 Mk4 (BP) 2019-Present.
For other models, please choose your vehicle here.
When the remote on a Mazda 3 Mk4 (BP, 2019–present) makes you stand right next to the door before it will lock, or the push-button start throws up a “key not detected” message, the culprit is almost always the little coin cell inside the fob rather than anything wrong with the car. This fourth-generation Mazda3 — hatchback and saloon — ships with Mazda’s Advanced Keyless smart fob across most trims, the slim black remote with a pop-out emergency blade. One thing catches a lot of owners out: unlike most rivals, this fob does not take the ubiquitous CR2032. Here is the exact cell it uses, how to open the case without marking it, and why a genuinely new key is still a dealer job.
How to Tell the Fob Battery Is Dying
- You have to be closer to the car. Range that used to reach across a car park shrinks to an arm’s length — the earliest and most reliable sign.
- “Key battery low” in the driver display. The Mazda3 shows a low key-battery warning in the instrument cluster message area when the fob’s cell is weakening; don’t ignore it.
- Push-button start hesitates. The car may not sense the key unless you hold the fob against the start button or the steering column — a documented workaround that itself signals a flat cell.
- Cold-morning dropouts. A tired lithium cell sags further in low temperatures, so the fob behaves in the cold and recovers when it warms up.
A fob cell in this Mazda3 usually lasts two to four years. If yours is around that age and doing any of the above, fit a fresh battery before you start suspecting the receiver or the immobiliser.
The Battery You Need
The Mazda 3 Mk4 smart fob takes a single CR2025 3-volt lithium coin cell — Mazda’s own owner’s manual specifies “CR2025 or equivalent”. This matters: a CR2032 is the same 20 mm diameter but thicker (3.2 mm versus 2.5 mm), and while it may seem to fit it can bow the contacts or refuse to let the cover seat. Buy the exact code printed on the cell you remove, and choose a quality branded battery — cheap cells sag under the brief high current the transmitter draws and can give poor range even when “new”.
How to Change the Battery
- Slide out the auxiliary key. Press the small knob on the back of the fob and pull the metal emergency blade straight out.
- Protect the case. Wrap the tip of a small flat-blade screwdriver in a strip of tape so it can’t scratch the glossy plastic.
- Open the cover. Insert the wrapped screwdriver into the slot the blade left behind and twist gently in the direction of the arrow moulded into the case, then slide and twist again to lift the cover away.
- Lift out the battery cap. Underneath the cover is a small rubber battery cap — take it off, and mind the O-ring seal that keeps moisture out; if it comes loose, set it aside to refit.
- Swap the cell. Note which way the old CR2025 sits, then lift it out by the edge. Fit the new cell with the positive (+) side facing up, exactly as Mazda specifies.
- Reassemble. Refit the rubber cap and the O-ring, press the cover back on until it clicks, and slide the auxiliary key home.
⚠️ Mind the polarity. Fit the CR2025 the wrong way up and the fob won’t transmit — and a reversed cell can quietly drain against the contacts. If you’re unsure, copy the orientation of the battery you just removed; on this Mazda3 that is positive-side up.
Did It Work?
Step back a few metres and lock, then unlock, the car with the buttons. Full range should return at once. Any “key battery low” message clears once the car next reads a healthy fob — sometimes immediately, sometimes after a short drive. Press the start button as normal to confirm keyless detection is back.
If a Fresh Cell Doesn’t Fix It
- Re-check the cell and its orientation. Confirm you fitted a CR2025 (not a thinner or thicker lookalike) and that it’s positive-side up and sitting flat under its contact.
- Clean the contacts. A film of finger grease or a fleck of corrosion on the terminals is enough to cause dropouts — wipe them with a dry cloth and handle the new cell by the rim.
- Try the emergency start. Hold the fob directly against the engine start button; if the car then starts, the transmitter is weak but the immobiliser transponder still works — usually a battery or fob-electronics issue, not the car.
- Rule out the car’s 12 V battery. A weak vehicle battery can make the whole keyless system sluggish and mimic a fob fault; if the car is generally slow to wake, have the 12 V battery tested.
New Key or Spare — Why It’s a Dealer Job
Changing the coin cell leaves the fob’s existing pairing untouched, so nothing needs reprogramming and the key works straight away. A replacement or additional smart key is a different matter: Mazda’s immobiliser has to be accessed with diagnostic equipment, blank fobs are ordered against the car’s VIN, and the mechanical blade must be cut to the lock. That makes adding a spare or replacing a lost key a job for a Mazda dealer or an established auto-locksmith. The cheap insurance is to keep the battery in your spare fresh too, so you’re never down to one working key.
Frequently Asked Questions
What battery does a Mazda 3 (2019 onwards) key fob take?
A single CR2025 3-volt lithium coin cell. It looks like the common CR2032 but is thinner — fit the CR2025 the manual calls for.
Can I use a CR2032 instead of a CR2025?
It’s not recommended — the CR2032 is thicker and can distort the contacts or stop the cover seating. Match the CR2025 code on the old cell.
Which way up does the battery go?
Positive (+) side facing up, per Mazda’s instructions. If in doubt, note the orientation of the old cell before you lift it out and copy it.
Do I need to reprogram the fob after changing the battery?
No. A battery swap doesn’t affect the fob’s coding, so it pairs and works immediately. Only a brand-new key needs programming.
How long should the fob battery last?
Typically two to four years. Leaving the fob sitting close to the car, so it wakes the keyless system constantly, tends to shorten that.
Can I still start the car if the fob battery is completely dead?
Yes. Use the auxiliary blade to unlock the door, then hold the fob against the engine start button as you press it — the transponder inside still works, so a flat cell won’t strand you.
If the remote died suddenly the moment the car’s 12 V battery was disconnected — rather than fading over weeks — the pairing, not the coin cell, may be the issue; check any stored fault code first on autodtcs.com.
Independent guide — not affiliated with Mazda. Always follow your official service manual and safety precautions. Read our full disclaimer.