Replace the Key Fob Battery

When the remote on a Mazda 6 (GJ/GL, 2012–present) only locks from arm’s length, needs a second or third press, or stops the push-button start recognising the key, the coin cell inside has almost always run flat. Most Mazda 6 of this generation carry the Advanced Keyless Entry & Start smart key — the rounded fob with a hidden metal auxiliary blade — while some lower-trim cars came with a plain flip-style remote. Both use the same battery, and Mazda 6 fobs have one detail owners miss: a small rubber sealing ring inside the case that keeps moisture out and must be seated correctly when you close it up. Here is the exact cell, the opening method, and why a fresh battery fixes almost every “dead key” scare.

How to Tell the Fob Battery Is Dying

• Shrinking range. You have to be right beside the car, or almost touching the door handle, before it locks or unlocks. This is the first and clearest warning.
• The car won’t detect the key. On a keyless Mazda 6, pressing Start with the fob in your pocket does nothing, or the dash shows a key-not-detected prompt — a weak cell can no longer be read from inside the cabin.
• “Key fob battery low” message. The Mazda 6 instrument display shows a low key-battery warning when the smart key voltage drops, often the earliest sign of all.
• Cold-morning dropouts. A cell that is nearly done struggles in low temperatures, so the remote may work by afternoon but not first thing.

A coin cell in a Mazda 6 fob usually lasts two to four years. If yours is around that age and doing any of the above, fit a fresh battery before you suspect the receiver, the immobiliser, or the car’s own 12 V battery.

The Battery You Need

The Mazda 6 GJ/GL key fob — both the Advanced Keyless smart key and the flip remote — takes a single CR2025 3-volt lithium coin cell. That is the spec in Mazda’s own owner’s manual, which says to replace it “only with the same type battery (CR2025 or equivalent).” It is a widely stocked cell, sold in supermarkets and hardware shops. Note the code carefully: the CR2025 is thinner than the more common CR2032. A CR2032 will physically wedge into many Mazda fobs, but it can strain the contacts and case, so stick to the CR2025 the fob was designed around. Buy a good branded cell — cheap ones sag under the brief current the transmitter draws and can give short range even when new.

Method 1 — Advanced Keyless Smart Key

1. Release the auxiliary key. Press the small knob (release catch) on the fob and slide the metal auxiliary blade straight out. This blade is your leverage tool and your way into a locked car if the fob is completely dead.
2. Open the case. Wrap a small flat-blade screwdriver in tape to avoid marking the plastic, insert it into the slot the blade left behind, and twist gently to start the seam. Work the tool along and prise the two halves apart.
3. Lift out the battery cap, then the cell. Note that the old CR2025 sits positive (+) side up — Mazda’s manual specifies fitting the new one with the positive pole facing up. Handle the cell by its edge; skin oils on the faces cause dropouts.
4. Check the rubber sealing ring. A soft rubber O-ring runs around the case to keep water out. Make sure it is still seated in its groove and undamaged; if it has slipped or come loose, reseat it before closing the fob.
5. Close and reassemble. Press the halves together until they click, then slide the auxiliary blade back in until it latches.

Method 2 — Flip / Remote Key (lower-trim cars)

1. Fold the metal blade closed so the fob is easier to grip.
2. Look for a seam or a small retaining screw on the back. If there is a screw, undo it with a fine cross-head (Phillips) driver; if not, prise the seam open gently with a taped flat blade.
3. Lift out the old CR2025, noting its orientation, and drop the fresh cell in the same way up.
4. Clip the case shut (or refit the screw — snug, not overtight, the housing is plastic) and test every button.

⚠️ Get the polarity right and mind the seal. Fitting the cell upside down can leave the key unresponsive, so always copy the orientation of the battery you removed — positive up on the smart key. And do not skip the rubber ring: a pinched or missing seal lets moisture into a fob that lives in a pocket or bag, which is a far more common killer of Mazda smart keys than the battery itself.

Confirm It Worked

Stand back a few metres and lock, then unlock, the car. Full range should return at once. For the keyless system, get in and press Start with the fob in your pocket — if it recognises the key first time, you are done. Any “key fob battery low” message usually clears the next time the car reads a healthy fob, occasionally after one drive rather than instantly.

If a Fresh Cell Doesn’t Fix It

• Re-check polarity and seating. Positive side up on the smart key, and the cell must sit firmly under its clip against clean contacts. A cell that is a hair proud of its cap gives intermittent response.
• Wipe the contacts. Finger grease or light corrosion on the terminals is enough to cause dropouts; clean them with a dry cloth or a cotton bud.
• Confirm the cell code. A too-thin or off-brand cell may not press hard enough on the contacts — make sure it really is a CR2025 and that it reads 3 V.
• Rule out the car. A weak vehicle 12 V battery can mimic key faults on a keyless Mazda 6 — if the car is also slow to wake or reluctant to start, have the main battery tested.

New Key or Spare — Why It’s a Dealer Job

Changing the coin cell keeps the fob’s existing coding, so nothing needs reprogramming and the remote works straight away. Cutting or coding a new key is a different matter: the Mazda 6’s immobiliser has to be accessed with diagnostic equipment, and blank smart keys must be ordered and matched against the car’s VIN. That makes adding a spare or replacing a lost key a job for a Mazda dealer or an auto-locksmith. The cheap insurance is to keep the battery in your spare key fresh too, so you are never down to a single working fob.

Frequently Asked Questions

What battery does a Mazda 6 (2012–present) key fob take?

A single CR2025 3-volt lithium coin cell, as specified in Mazda’s owner’s manual. The same cell fits both the Advanced Keyless smart key and the flip-style remote.

Can I use a CR2032 instead of a CR2025?

The CR2032 is the same diameter and voltage but noticeably thicker. It may physically fit, yet it can strain the fob’s contacts and case, so use the CR2025 the key was designed for whenever you can.

Which way up does the battery go?

On the smart key the cell sits positive (+) side up, matching Mazda’s instruction to insert it with the positive pole facing up. Always copy the orientation of the cell you removed.

What is the little rubber ring inside the fob?

It is a sealing O-ring that keeps moisture out of the electronics. Make sure it stays in its groove when you change the battery; a missing or pinched seal lets water in and can kill the fob.

Do I need to reprogram the key after changing the battery?

No. A battery swap does not affect the coding, so the fob works immediately. Only a brand-new key needs programming to the car.

Can I still drive if the fob is completely dead?

Yes. Use the auxiliary blade hidden in the fob to unlock the driver’s door, then hold the fob against the Start button (or as directed in the manual) to start the car — the keyless system is designed to work with a flat fob battery, so a dead cell won’t strand you.

If the remote died suddenly right after the car’s 12 V battery was disconnected — rather than fading gradually — the cause is more likely the electronics losing sync than the coin cell, and a stored fault code can point you to it. You can look up what any logged code means on autodtcs.com.