Start the Car With a Dead Key Fob Battery

The Lexus UX (2019–present) — UX 200, UX 250h hybrid and the electric UX 300e — relies on Smart Access keyless entry and a push-button start. When the smart key’s coin cell goes flat, the proximity sensing and remote buttons stop responding, but the car can still read the key’s passive chip at point-blank range. This guide shows a UX owner how to start the car and unlock the doors with a dead fob, then how to fit a fresh coin cell so it does not happen again.

Start the UX with a dead fob battery

The UX has a backup antenna built into the START STOP switch that can wake the key’s transponder when the fob is held against it:

1. Sit in the driver’s seat, close the door, and press the brake pedal firmly.
2. Hold the smart key with the Lexus “L” emblem facing the START STOP button.
3. Touch the emblem side of the fob directly to the button. A beep confirms the car has read the chip and the button indicator changes.
4. Keeping your foot on the brake, press the START STOP button. The UX 200 starts its engine; the UX 250h and UX 300e show READY.

If nothing happens, hold the fob flat against the button and try again — the read zone is small.

Unlock the doors with the mechanical key

A dead fob will not trigger the touch-sensitive door handles, so get in mechanically:

1. Slide the release latch on the back of the smart key and pull out the hidden metal emergency key.
2. At the driver’s door, lift off the small cover at the rear of the door handle with the blade to reveal the keyhole.
3. Insert the blade, turn to unlock, and open the door. If the alarm sounds, touching the fob to the start button and switching to ON silences it.
4. Slide the metal key back into the fob so it is not lost.

Why touching the fob to the button works

The Smart Access fob contains a passive immobiliser chip that needs no power of its own — an induced magnetic field wakes it. The UX’s START STOP button houses a short-range antenna; held against the fob emblem, it energises the chip just long enough for the car to verify the correct key. That is the factory-designed fallback, which is why it works even when the coin cell is completely dead.

Replace the smart key battery

The UX smart key uses a CR2032 lithium coin cell (check the marking on the old one to be sure). To replace it:

1. Slide out the metal emergency key.
2. Insert a coin or thin flat-blade screwdriver into the slot the key left and twist gently to split the case.
3. Lift out the old CR2032 — the “+” face points toward the back cover.
4. Press in the new cell, close the case, and reinsert the metal key.
5. Test the lock buttons and proximity sensing before relying on the fob.

A coin cell lasts about one to two years. The UX shows a “key battery low” message on the display when it is getting weak.

Frequently Asked Questions

Does the electric UX 300e start the same way? Yes. Touch the fob to the button, press the brake, and press START; the car shows READY since there is no engine to crank.

Where is the keyhole on the UX driver’s door? Behind a small removable cap at the rear of the driver’s door handle — pry it off with the metal blade.

Will manual unlocking trigger the alarm? It can. Touch the fob to the START STOP button and switch to ON to clear it.

Which battery does the UX key use? A CR2032 in most cases; always confirm against the cell you remove.

Is it safe to keep driving after a touch-start? Yes, the car runs normally. But the dead fob still cannot operate the doors, so change the cell soon.

If a dashboard warning light stayed on afterward, you can look up the meaning of any trouble code at autodtcs.com.