These instructions apply to the Lexus RX Mk2 (XU30) 2003-2008.
For other models, please choose your vehicle here.
The second-generation Lexus RX (XU30, 2003–2008 — the RX330, then RX350, and the hybrid RX400h) shipped with two very different keys, and which one you hold decides both the battery it takes and how you open it. Lower and earlier cars carry a remote-head (“combo”) key — one moulded unit with the blade and buttons together — while cars fitted with Smart Access (standard on the RX400h, optional on the petrol cars) carry a separate smart keyless fob with a fold-out emergency blade. If your remote only works up close, flashes weakly, or has stopped altogether, the coin cell inside is almost certainly flat. Below is the exact cell for each key, how to open it without cracking the case, and why a spare key is still a dealer job.
How to Tell the Fob Battery Is Dying
- Shrinking range. You have to be right beside the RX — or press the fob against the door handle — before it locks or unlocks. This is the earliest and most reliable sign.
- Weak or dead indicator light. The LED on the remote-head key stops flashing brightly, or at all, once the cell voltage sags.
- Smart-entry drop-outs. On a Smart Access RX the door handle stops recognising the fob in your pocket, and you fall back to the button or the blade.
- Cold-morning failures. A nearly flat cell struggles most in the cold, so intermittent response on chilly mornings usually means the battery, not the car.
A coin cell in an RX key typically lasts two to four years. If yours is older than that and behaving this way, fit a fresh cell before you start suspecting the receiver or the immobiliser.
The Battery You Need
It depends on which key you have, and they are not the same:
- Remote-head / combo key (RX330, and base RX350) — a CR1616 3-volt lithium coin cell. Note that some printed material and parts listings quote a CR2016 for these cars; the CR1616 is the thinner cell that actually fits most of these remotes. Because that discrepancy exists, always check the code stamped on the cell you take out and match it.
- Smart Access fob (RX400h, and Smart-equipped petrol cars) — a CR1632 3-volt lithium coin cell (the same cell Lexus uses in its OEM smart-key battery part for this era).
Buy a good-quality branded cell in either case — cheap coin cells sag under the short, sharp current a transmitter draws and can give poor range even when brand new.
Method 1 — Remote-Head (Combo) Key
- Look at the seam that runs around the edge of the plastic head, above the metal blade. Some of these keys have a tiny cross-head screw holding the halves together — if yours does, undo it first with a jeweller’s Phillips driver and keep the screw somewhere safe, as it is easy to lose.
- Insert a thin flat tool — a small flat-blade screwdriver or a plastic pry tool — into the seam and gently twist to split the two halves of the head. Work around the edge rather than forcing one spot.
- Note the way the old CR1616 sits, then ease it out. Fit the new cell the same way up — on these Lexus remotes the positive (+) face normally sits upward, but copy whatever you removed.
- Press the halves back together until the seam clicks shut, refit the screw if there was one, and test every button.
Method 2 — Smart Access Fob (RX400h and Smart-equipped cars)
- Release the emergency blade: slide or press the catch on the fob and pull the mechanical key blade straight out. It leaves a slot in the body of the fob.
- Put the tip of a flat-blade screwdriver into that slot — the space the blade came out of — and apply gentle leverage to pop the cover plate off the fob.
- You may find a small electronic module sitting over the battery; lift that out of the way first, then prise the old CR1632 free.
- Seat the new cell exactly as the old one sat — Lexus dealer guidance for these smart keys is positive (+) side up, but always match the orientation you removed. Refit the module, click the cover plate back on, and slide the emergency blade home until it latches.
⚠️ Get the polarity right. A coin cell fitted the wrong way round will not power the transmitter and can slowly discharge itself, leaving you with a “new” battery that still won’t work — so always copy the orientation of the cell you took out rather than guessing.
Confirm It Worked
Stand back a few metres and lock, then unlock, the RX. Full range should return straight away. On a Smart Access car, also confirm the door handle senses the fob again and that the push-button start reads it normally — if it does, the swap is done.
If a Fresh Cell Doesn’t Fix It
- Re-check the cell code and orientation. A CR1616 and a CR1632 are different thicknesses; the wrong one sits loosely on the contacts or won’t let the case close. Confirm you fitted the correct cell the correct way up.
- Clean the contacts. Finger grease or light corrosion on the terminals is enough to cause drop-outs — wipe them with a dry cloth and reseat the cell firmly under its clip.
- Try the other key. If your second key works normally with its own battery, the problem is the first fob (a tired transmitter), not the car’s receiver.
- Rule out the car’s own battery. A weak 12-volt battery can make an RX slow to wake and mimic key faults; if the car itself is sluggish to respond, have the vehicle battery checked.
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 key works the moment you close the case. Cutting or coding a new key is different: the RX’s immobiliser must be accessed with diagnostic equipment, and blank smart fobs have to be ordered against the car’s VIN and registered to it. That makes a spare or a lost-key replacement a job for a Lexus dealer or a qualified auto-locksmith — so keep the battery in your spare fresh too, and you’re never down to one working key.
Frequently Asked Questions
What battery does a 2003–2008 Lexus RX key take?
It depends on the key. The remote-head (combo) key used on the RX330 and base RX350 takes a CR1616; the Smart Access fob used on the RX400h and smart-equipped cars takes a CR1632. Check the code on the old cell before buying, as some listings mistakenly quote a CR2016 for the remote-head key.
Which way up does the cell go?
Match the orientation of the cell you remove. On the smart fob, Lexus dealer guidance is positive (+) side up; on the remote-head key the positive face also normally sits upward — but copying what you took out is the only sure way, and fitting it upside down will leave the key dead.
Do I need to reprogram the key after changing the battery?
No. A battery swap doesn’t affect the fob’s coding or its pairing with the car, so it works immediately. Only a brand-new key needs programming to the immobiliser.
How long should an RX key battery last?
Usually two to four years. A Smart Access fob that constantly wakes because it sits near the car can run down a little faster than a button-only remote-head key.
The key still won’t work after a new battery — is the car faulty?
Rarely. Re-check that you fitted the right cell (CR1616 vs CR1632) the right way up, then clean the contacts. If a known-good cell still won’t light the LED or trigger the locks, the fob’s transmitter may have failed — but that is uncommon next to a simple flat or upside-down cell.
Can I drive the RX if the fob is completely dead?
Yes. Use the metal emergency blade to unlock the driver’s door. On a Smart Access RX, the push-button start is designed to still read the fob at close range with a flat battery — typically by holding the fob against the start button — so a dead cell won’t strand you.
If your remote stopped responding suddenly right after the 12-volt battery was disconnected — rather than fading gradually — the cause is more likely the electrical system than the coin cell, and the car may have logged a fault; you can look up any stored code on autodtcs.com.
Independent guide — not affiliated with Lexus. Always follow your official service manual and safety precautions. Read our full disclaimer.