Start the Car With a Dead Key Fob Battery

Last updated: June 12, 2026

The Hyundai Tucson (Mk3/TL, 2015–2020) was sold with the Smart Key proximity fob and a START/STOP button on most trims, while entry specs kept a conventional turn-key barrel. When the fob’s CR2032 coin cell goes flat the remote-lock buttons and keyless sensing stop, but the Tucson can still read the key at point-blank range and start normally. A flat fob is rarely a no-start — it is almost always just a dead coin cell.

Start a Smart Key Tucson with a dead fob

If your Tucson has the keyless Smart Key and a START/STOP button on the dash:

1. Get into the car using the emergency blade first (next section) and sit in the driver’s seat.
2. Press and hold the brake pedal down firmly.
3. Hold the Smart Key so the back face of the fob touches the START/STOP button directly.
4. With the fob pressed against it, push the START/STOP button.
5. The immobiliser reads the transponder through the button and the engine cranks and starts.

Turn-key Tucson — the blade still starts it

Entry trims came with a flip key and an ignition barrel. A flat coin cell only kills the remote-lock buttons; unlock the driver’s door with the blade, slide it into the barrel and start the car as usual. The transponder in the key head is energised by the barrel, so it works regardless of the cell.

Unlock the Tucson with the emergency key blade

1. Slide the catch on the back of the Smart Key and pull the metal emergency blade out of the body.
2. At the driver’s door handle, prise off the small cover cap to expose the lock slot, then insert the blade and turn to unlock.
3. The alarm may chirp until the car recognises the key — starting the engine cancels it.

Why a flat fob still starts the Tucson

The coin cell only powers the remote buttons and the proximity broadcast that lets you unlock and start without touching the fob. The chip that authorises the engine is a passive RFID transponder with no battery of its own — it is woken by the coil behind the START/STOP button (or the ignition barrel on turn-key cars). Because that chip never depends on the cell, the immobiliser can still verify the key when the fob is completely dead.

Replace the fob battery

The Tucson Mk3 Smart Key uses one CR2032 3V lithium coin cell. Pull the emergency blade, then use it or a small coin to twist the case open at the seam. Lift out the old cell, fit the new one with the + side facing the same way, and click the halves back together. Remote locking and proximity start return immediately. A genuine Energizer or Duracell cell typically lasts two to three years.

Frequently Asked Questions

Where exactly do I hold the fob to start it?

Press the back of the Smart Key flat against the START/STOP button itself, keep the brake down, then push the button with the fob still in place. The Mk3 Tucson has no drop-in slot.

Does my Tucson have keyless start?

Most TL Tucsons came with the Smart Key and a START/STOP button. Entry cars use a flip key with an ignition barrel and just need the blade.

The remote stopped working but the car still starts — is that bad?

On a turn-key Tucson that is normal: a flat cell disables only remote locking while the key transponder still starts the engine. Replace the CR2032 when convenient.

Why did the alarm sound when I used the blade?

Mechanical entry looks like a forced entry until the transponder is read. Starting the engine clears the alarm.

Which battery does the Tucson Smart Key take?

A single CR2032 3V lithium coin cell, available from any supermarket or hardware shop.

If a warning light stayed on after the no-start, look it up on autodtcs.com to confirm it was just the fob and not an immobiliser fault.