Start the Car With a Dead Key Fob Battery

The Lexus RX (AL20, 2015–2022) — RX 200t/300, RX 350, the RX 450h hybrid and the long-wheelbase RX 450hL — uses Smart Access keyless entry with a push-button start. When the smart key’s coin cell goes flat, the door sensors and remote buttons stop responding and the dash flags a “Smart Key system” or “key battery low” message. Nothing has truly failed — the fob still carries a passive chip the car can read at very close range. This guide shows an RX owner how to start the engine and unlock the doors with a dead fob, then how to fit a fresh coin cell.

Start the RX With a Dead Fob

The fourth-generation RX reads the key’s chip through a short-range antenna built into the START/STOP switch, so you hold the fob to the button rather than wait for the hands-free link.

1. Unlock and get in first (mechanical-key steps below), then sit in the driver’s seat with the door closed.
2. Press the brake pedal fully and keep it held.
3. Touch the smart key flat against the ENGINE START STOP button, chrome Lexus “L” emblem facing the button. Hold it there until you hear a beep and the button indicator changes.
4. With the brake still down, press START. The RX 350 fires the engine; the RX 450h and 450hL hybrids power to READY.

If nothing happens on the first try, reposition so the emblem sits squarely over the button — the read zone is only a centimetre or two wide.

Unlock With the Mechanical Key

A flat fob will not trigger the touch-sensitive door handles, so the RX hides a metal blade inside the smart key.

1. Slide the release latch on the back of the key and pull the mechanical blade out.
2. At the driver’s door, the lock cylinder sits behind a small cover at the rear of the handle on the AL20. Prise the cover off with the blade tip to expose the keyhole.
3. Insert the blade and turn to unlock, then open the door. The alarm may sound; starting the car as above silences it.
4. Slide the metal blade back into the fob so it is not lost.

Why Touching the Key to the Button Works

Inside the Smart Access fob is a passive immobiliser chip that needs no power of its own — it is woken by an induced magnetic field. The START/STOP button on the RX contains a small antenna; held against the fob emblem, it energises that chip just long enough for the car to confirm the right key is present. That is the factory-designed backup, which is why it works even when the coin cell is completely flat.

Replace the CR2032 Coin Cell

The RX AL20 smart key runs on a single coin cell that lasts roughly one to two years.

1. Slide out the mechanical key to expose the case seam.
2. Insert a coin or small flat screwdriver into the slot the blade left and twist gently to part the two halves.
3. Lift out the old battery. The RX AL20 smart key takes a CR2032 3V lithium coin cell. Note the positive (+) face points up toward the back cover.
4. Press in the new cell the same way the old one sat, click the case shut, and reinsert the metal key.

Stand beside the car and press lock once; hands-free entry should behave normally again. If the “key battery low” message returns, change the cell promptly.

Frequently Asked Questions

My RX still won’t start after touching the key to the button. Wait for the beep and indicator change before pressing START, and make sure the brake is fully down. If there is no beep, try the other face of the key, or rest it in the centre console tray and press START — some AL20 cars read a second antenna there.

How do I know my RX 450h hybrid has started? The green READY light and a fully-lit display confirm it. The petrol engine may stay silent at first in hybrid mode — that is normal.

Where exactly is the door keyhole? Behind a removable cover at the rear of the driver’s door handle. The other doors have no barrel.

Which battery does the 2015–2022 RX key take? A CR2032 in most cases; always confirm against the marking on the cell you remove.

Could a weak 12V battery cause the same warning? Yes. If a fresh coin cell doesn’t restore keyless operation, or the car cranks slowly, suspect the main 12V battery (the RX 450h carries its 12V auxiliary in the boot area).

If a warning light stayed on after the dead-fob episode, you can look up the exact fault code on our sister site autodtcs.com to see whether it concerns the smart-key system or something else.