These instructions apply to the Lexus RX Mk4 (AL20) 2015-2022.
For other models, please choose your vehicle here.
If the smart key on your Lexus RX (2015–2022) has lost its range or shows a low-key-battery message, the coin cell inside is almost certainly flat. The fourth-generation RX (chassis code AL20, covering the RX 350, RX 450h and long-wheelbase RX 450h L) uses a proximity smart key that’s always awake, listening so it can unlock as you grip the handle and let you start with a button press. That steady chatter drains the little battery far quicker than an old lock-and-key remote. Look for a range that shrinks over a few weeks, a fob that only fires on the second or third press, needing to hold the key almost against the handle before it responds, or a “key battery low” note on the driver’s display. It isn’t the car or the key electronics failing — it’s the cell, and the swap takes about five minutes. Just check which of the two RX key types you have first, because they take different batteries.
How to Tell the Fob Battery Is Dying
- Shrinking range. You once locked the RX from across the car park; now it only answers from a step or two away.
- Works on the second or third press. A weak cell can still fire, just not reliably first time.
- Keyless only up close. Having to hold the fob near the handle or start button before it registers means the proximity signal is fading.
- “Key battery low” message. The RX shows this in the instrument display when it senses the cell is nearly done.
- Worse in the cold. A marginal coin cell plays up first on a frosty morning and recovers a little once it warms.
As a rough guide, a constantly-listening smart key like the RX’s tends to want a fresh cell closer to once a year, where a plain button-only remote might go two to four years. If yours has seen a couple of winters, it’s simply due.
The Battery You Need — Two Key Types
- Standard smart key (with a pull-out mechanical blade) — CR2032, a 3 V lithium coin cell.
- Card-style smart key (the flat card key) — CR2412, a taller, less common 3 V lithium cell — check before buying.
Both are 3 V lithium coin cells, but they are not interchangeable, so confirm which key you’re holding before you order. Fit a reputable branded cell (Panasonic, Sony/Murata, Duracell, Energizer) and don’t drop in a thinner substitute that “looks close” — a CR2025 where a CR2032 belongs sits loose and drops contact. The safest check of all: open the key, read the code stamped on the cell you take out, and match it exactly.
Replacing the Cell
Standard smart key (CR2032):
- Slide out the mechanical blade. Push and hold the small release knob on the side and pull the metal emergency key free of the fob.
- Separate the two halves of the case with a small screwdriver at the seam — work round the joint rather than forcing one point. Some keys use clips you’ll need to press back on reassembly.
- Note which way up the cell sits before you disturb it — the marked + face is normally uppermost. A quick phone photo makes a handy reference.
- Lift the old CR2032 out by its edge, and handle the replacement by its rim too — skin oil on the flat faces shortens its life.
- Fit the new CR2032 the same way up, positive side up, pressed fully home under the retaining clip.
- Close the case until it clicks all the way round, then push the mechanical blade back in until it locks.
Card key (CR2412): pull off the cover, lift out the old cell noting its orientation, fit a new CR2412 positive side up, and refit the cover.
⚠️ Polarity matters. Fitting the cell upside down won’t harm the key, but it simply won’t work — and it’s the commonest reason a “new” battery seems dead on arrival. Positive (the flat face with the writing) faces up unless the holder plainly shows otherwise.
Confirm It Worked
Stand back a few metres and lock, then unlock, the RX — the range should be back to normal. Walk up with the fob pocketed and try the door handle to confirm keyless entry, then press START with your foot on the brake to confirm the car reads the key. If a “key battery low” message was showing, it usually clears by itself after a drive cycle or two once the fresh cell is in.
If a Fresh Cell Doesn’t Help
- Re-check polarity and seating — positive up, the cell fully under the clip and not sitting proud.
- Confirm the right code — CR2032 for the standard key, CR2412 for the card key; a slightly-too-thin mismatch won’t make solid contact.
- Wipe the contacts inside the key with a dry cloth or cotton bud — oxide or grease can block an otherwise good cell.
- Use the backup start in the meantime: hold the fob flat against the START button as you press it and the car reads the key directly.
- Rule out the car’s 12 V battery. If the RX is also slow to wake or the dash flickers, a tired vehicle battery can imitate key faults.
- Still flagging a fault? A smart-entry problem can store a code — look it up on autodtcs.com.
New Key or Spare — Why It’s a Dealer Job
Swapping the coin cell keeps all the existing coding, so the key works exactly as before — there’s nothing to reprogram. A new or replacement key is different: it has to be programmed to your car’s VIN through the immobiliser with dealer or auto-locksmith diagnostic equipment, and there is no owner button-sequence for it on the RX. If you keep a spare key in a drawer, change its cell every few years too, so it’s ready when you need it rather than dead when you reach for it.
Frequently Asked Questions
What battery does the Lexus RX key use?
The standard smart key (with a pull-out mechanical blade) takes a CR2032 3 V lithium coin cell. The flat card-style key takes a CR2412. Both are 3 V lithium but not interchangeable — check which key you have, or read the code stamped on the cell you remove.
Which way up does the battery go?
Positive side up in almost every case — the flat face carrying the writing and the CR2032 or CR2412 marking. Note the orientation before lifting the old cell out and fit the new one to match. Fitting it upside down is the number-one reason a fresh battery seems dead.
Do I need to reprogram the key after changing the battery?
No. A battery swap keeps all the existing coding, so the key locks, unlocks and starts the RX exactly as before. Programming is only required for a brand-new or replacement key that has never been paired to your car.
How long should the new battery last?
Because the RX smart key is always listening for the car, expect roughly a year or so from a fresh cell — less if the key lives near the car overnight, a little more if it’s stored away. A button-only remote lasts longer, but a proximity smart key drains faster by design.
The key still doesn’t work after a new battery — now what?
Re-check the cell is the correct code, the right way up, and fully seated, and clean the contacts inside the key. Make sure you didn’t fit a too-thin substitute. If it still won’t respond, the car’s 12 V battery or a smart-entry fault may be involved — use the backup start and have it checked.
Can I still drive if the fob battery is completely dead?
Yes. Use the mechanical blade inside the standard key to unlock the driver’s door, then hold the fob flat against the START button as you press it — the car reads the transponder directly and starts even with a flat coin cell. A dead key battery alone never strands you.
If the key flagged low after a battery disconnection, see our RX battery disconnect/reconnect guide. For any stored fault code, look it up on autodtcs.com.
Independent guide — not affiliated with Lexus. Always follow your official service manual and safety precautions. Read our full disclaimer.