Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

The Toyota Yaris (Mk4/XP210, 2020–Present) is the GA-B-platform supermini, sold widely as a hybrid, with Smart Entry and push-button start on mid and upper grades and a bladed remote key on entry models. When the fob coin cell dies, the keyless link drops and the dash shows a key warning — but the XP210 keeps a simple fallback that gets you moving. Here is the Yaris procedure.

Push-Button Yaris: Start It With a Dead Fob

The car can still read the chip inside the fob at point-blank range even with a flat battery, so you bring the fob to the button.

1. Unlock and get in (mechanical-key steps below), then sit in the driver’s seat with the door shut.
2. Press the brake pedal fully and hold it.
3. Touch the fob flat against the START/STOP button, badge face to the button. Wait for the confirmation beep.
4. With the brake still down, press START. The hybrid powers up to READY.

As a hybrid there may be no engine crank — the green READY light and a fully-lit display confirm it has started. No beep? Turn the fob over and try the other face squarely against the button.

Unlock With the Hidden Mechanical Key

A dead fob won’t operate the door buttons, so use the blade inside the Smart Key.

1. Press the release on the fob and slide the metal key out.
2. At the driver’s door, the lock cylinder sits behind a small cover on the handle. Lever the cover off with the blade tip to reveal the keyhole.
3. Insert and turn to unlock. The alarm may chirp; starting the car clears it.

Entry-grade XP210s with the bladed remote key still turn-start from the door key if so equipped, or simply lose remote locking only — check which key you hold.

Why the Touch-to-Button Trick Works

Behind the start button is a short-range induction antenna that powers and reads the passive transponder in the key — a chip with no battery of its own. Only the longer-range radio for hands-free unlocking relies on the coin cell, so a dead battery leaves the close-range immobiliser handshake working. That is the limp-home path Toyota designs into every Smart Key car.

Replace the Coin Cell

Change the battery the same day so hands-free entry returns.

1. Slide out the mechanical key to expose the case seam.
2. Twist a coin in the slot to split the fob halves.
3. Check the old cell before buying: most XP210 smart fobs take a CR2032, but some Yaris fobs use the thinner CR2025 — both are 3V lithium, and the print on the old battery confirms which. Fit the new one positive (+) side the same way up.
4. Handle the cell by its edge, then click the case shut.

Stand by the car and press lock once; the buttons should respond again.

Frequently Asked Questions

Is my Yaris fob a CR2032 or CR2025? It depends on the fob version. Open it and read the number on the old coin cell — that is the reliable way. Don’t force a thicker CR2032 into a holder made for a CR2025.

How do I know the hybrid has started? The green READY light comes on and the instruments fully illuminate. The petrol engine may stay silent at first — normal for a hybrid.

Where is the door keyhole? Under a small cap at the rear of the driver’s door handle; the passenger doors have no barrel.

A key warning stayed on after a new battery. The same message can come from a weak 12V battery. If a fresh coin cell doesn’t restore keyless entry, have the 12V tested.

Can the metal key start the car? On a Smart Entry Yaris, no — the blade only unlocks the driver’s door. Starting is via the button.

If a warning light lingered after you drove off, our sister site autodtcs.com can decode the stored fault so you know whether it relates to the smart-key system.