Start the Car With a Dead Key Fob Battery

The Lexus CT 200h (A10, 2011–2022) is a full hybrid that uses Smart Access keyless entry with a push-button start. When the smart key’s coin cell dies, the proximity handles and remote buttons stop responding and the multi-information display shows a “key battery low” or “Smart Key system” warning. Nothing has actually failed — the fob still holds a passive chip the car can read at point-blank range. This guide walks a CT owner through starting the car and unlocking the doors with a dead fob, then through fitting a fresh coin cell.

Start the CT With a Dead Fob

The CT 200h 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. Because it is a full hybrid, the system powers up to READY rather than cranking an engine.

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 CT powers to READY — the green READY light confirms the car is on, even though the petrol engine may stay silent.

If it does not catch the first time, 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 CT 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 A10. 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; switching the car to READY 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 CT 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 Coin Cell

The CT 200h 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 early CT 200h smart key commonly takes a CR1632 3V lithium coin cell rather than the more usual CR2032 — always check the marking on the cell you remove before buying. 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.

Test the lock buttons and proximity sensing before you rely on the fob again. If the “key battery low” message returns, replace the cell promptly.

Frequently Asked Questions

Which battery does the CT 200h key take? The early A10 smart key commonly uses a CR1632, not the CR2032 fitted to most later Lexus keys. Always confirm against the marking on the cell you remove — fitting the wrong size will not seat correctly.

How do I know the CT has started? The green READY light and a fully-lit display confirm it. As a hybrid, the petrol engine may stay silent at first — that is normal, the car is on and ready to drive.

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.

Will manual unlocking set off the alarm? It can. Switching the car to READY by touching the fob to the button clears it.

Could a weak 12V battery cause the same warning? Yes. If a fresh coin cell doesn’t restore keyless operation, or the dash behaves oddly, suspect the main 12V battery (the CT 200h carries its 12V auxiliary in the boot).

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.