Replace the Key Fob Battery

When the Smart Key on your Toyota Mirai (JPD10, 2015–2020) starts feeling unreliable — you have to stand close to lock or unlock, the car only responds on the second or third press, or a “key battery low” message appears on the display — the cause is almost always the coin cell inside the fob, not a fault with the hydrogen fuel-cell car itself. The Mirai uses Toyota’s Smart Entry system, and because a keyless fob is always listening for the car it drains its cell faster than a plain button-only remote would. The fix is easy: the Mirai key uses a CR2032 cell, it costs very little, and the swap takes a couple of minutes. Here’s how.

How to Tell the Fob Battery Is Dying

A coin cell fades gradually, so the warning signs usually build up over a week or two rather than appearing overnight:

• Shrinking range — you used to lock from across the car park; now the car only responds within a metre or two.
• A “key battery low” message on the dash, which the Mirai shows when the fob’s signal weakens.
• Smart Entry only works up close — walking up and touching the handle no longer wakes the car, so you resort to pressing the buttons.
• Intermittent response that’s worse in cold weather, because a tired lithium cell loses voltage further when it’s cold.

As a rough guide, a keyless Smart Key like the Mirai’s tends to want a fresh cell nearer once a year, while a plain button remote can go two to four years. If yours is a couple of years old and doing any of the above, the battery is the first and cheapest thing to rule out.

The Battery You Need

The Mirai Smart Key takes a single CR2032 — a 3 V lithium coin cell roughly 20 mm across and 3.2 mm thick, one of the most common cells sold anywhere. Buy a branded one; a good lithium cell holds its voltage far more steadily over time than a cheap multipack. Don’t substitute a thinner cell such as a CR2025 — it may look close but won’t press firmly against the contacts, giving weak range from day one. If in doubt, the code is stamped on the flat face of the cell you remove; match it exactly.

What You Need

• One fresh CR2032 3 V lithium coin cell.
• A small flat-blade screwdriver (tape the tip to avoid marking the case).
• Clean, dry hands.

How to Replace the Mirai Key Battery

1. Push and hold the release knob and pull the mechanical (emergency) key out of the fob.
2. Insert the screwdriver into the slot the key leaves and gently remove the cover — work around the seam and don’t use excessive force.
3. Note which way up the old cell sits, then lift out the old CR2032, avoiding touching the terminals with your fingers.
4. Fit the new cell with the positive (+) side facing up, exactly as the old one sat, handling it by the edges.
5. Refit the cover until the seam closes evenly and slide the mechanical key back in.
6. Test lock/unlock and Smart Entry from a normal distance.

⚠ Polarity matters. A cell fitted upside-down won’t power the fob and can bridge the contacts in some holders. If you’re unsure, take a quick photo before lifting the old cell out — positive (+) side up is correct on the Mirai key.

Check It Worked

Stand a few metres back and lock, then unlock, watching for the indicators to flash. Walk up and try Smart Entry by touching the door handle. If a “key battery low” message was showing, it usually clears itself once the car sees a strong signal again; a short drive cycle guarantees the reset.

If a Fresh Cell Doesn’t Fix It

• Re-open the fob and confirm the cell is the right way up and fully seated.
• Check you fitted a CR2032 and not a thinner look-alike.
• Wipe the contacts with a dry cloth or cotton bud — a fingerprint or fleck of corrosion is enough to cause weak range.
• If Smart Entry is dead, hold the fob flat against the START/STOP button and press to power up on the backup transponder; the car works normally while you sort the fob.
• If everything checks out and it’s still weak, don’t overlook the car’s own 12 V battery — a flat vehicle battery can mimic key faults.

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

Changing the coin cell never disturbs the fob’s coding, so your key works the moment the new battery is in. A brand-new or replacement key is a different matter: it must be matched to the car’s immobiliser and VIN with diagnostic equipment, which means Toyota or an auto locksmith. Keep the battery in your spare key fresh too — a spare left in a drawer for a couple of years is often flat exactly when you finally need it.

Frequently Asked Questions

What battery does the Mirai key take?

A single CR2032 3 V lithium coin cell — a common, inexpensive battery sold in most supermarkets and hardware shops. Choose a branded lithium one and avoid thinner substitutes such as the CR2025.

Which way up does the cell go?

Positive (+) side facing up, matching the cell you removed. If unsure, photograph the old cell before lifting it out so you can copy its orientation exactly.

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

No. Replacing the coin cell doesn’t affect the fob’s pairing with the car, so it works immediately. Reprogramming is only needed for a genuinely new or replacement key.

How long should a new key battery last?

Because the Mirai Smart Key is a keyless fob that’s always listening for the car, expect roughly a year or so from a fresh cell — less than a plain button-only remote. If it fades much sooner, re-check the cell code and seating.

My key still won’t work with a fresh cell — now what?

Re-open it and check the cell orientation and that the contacts are clean and dry. If it’s still dead, the fob may have an internal fault and need replacing and coding by Toyota — and remember a flat 12 V car battery can look like a key problem too.

Will the car start if the key battery dies?

Yes. Hold the fob against the START/STOP button (Toyota marks the spot) and press to power up using the backup transponder. The mechanical key inside the fob unlocks the driver’s door if central locking won’t respond.

To clear the maintenance reminder after a service, see our Toyota Mirai service indicator reset guide. For warning lights, look up the code on autodtcs.com.