Start the Car With a Dead Key Fob Battery

Last updated: June 12, 2026

The Honda Jazz (Mk3/GK, 2015–2020) was sold in Europe with two ignition systems: the EX and higher trims carry Honda Smart Entry with a red push-button start, while S and SE grades keep a conventional bladed turn-key in the column. If your key has stopped working, the recovery depends on which one your Jazz has — and on a Smart Entry car the fix takes seconds.

Start a Smart Entry Jazz With a Dead Fob

On a push-button Mk3 the fob normally talks to the car over a short radio link. When the coin cell dies the link drops, the dash flags a low key-battery warning and pressing START does nothing. The immobiliser can still read the transponder chip inside the fob at very close range, so you bring the fob to the button.

1. Get into the car first using the emergency key (steps below) and sit in the driver’s seat.
2. Press the brake pedal fully and hold it down.
3. Touch the back of the smart key — the flat side opposite the buttons, with the H badge — against the START/STOP button.
4. Press the button while the fob is held there. The engine starts. If it doesn’t catch first time, press START twice in quick succession with the fob still touching it.

The coil behind the button only reaches a centimetre or two, so square the flat back of the fob over the switch if the first attempt fails.

Unlock the Door With the Emergency Key

A flat fob won’t pop the doors by radio, so the Smart Entry fob hides a metal blade.

1. Press the release catch near the key-ring loop and slide the emergency key out of the fob.
2. At the driver’s door the lock cylinder is in the handle; on the Mk3 it usually sits under a slim cap — lever the cap off with the blade tip to reveal the keyhole.
3. Insert the key and turn to unlock. The alarm may chirp — starting the car clears it.

Turn-key Jazz models are simpler: the same blade unlocks the door and turns the ignition, so a flat fob battery only kills the remote-lock buttons — the car starts as it always has.

Why Touching the Fob to the Button Works

Behind the START switch sits a small induction antenna. The immobiliser energises it and reads the passive transponder chip in the key — the same chip a dealer programmer talks to. The chip needs no battery; it draws power from the antenna’s field. So even a completely flat coin cell leaves the security handshake intact, which is exactly why Honda builds this backup into every Smart Entry Jazz.

Replace the Coin Cell

Get moving with the trick above, then change the battery the same day.

1. Slide the emergency key out to expose the seam in the fob.
2. Twist the two halves apart with the slot the key leaves, or a coin.
3. Most Mk3 Smart Entry fobs take a CR2032 3V lithium cell; the slimmer remote on some cars uses a CR1620. Check the marking on the spent cell before buying.
4. Fit the new cell the same way up as the old one, avoid touching both faces with bare fingers, and clip the halves together until they click.

No reprogramming is needed; the buttons work again immediately.

Frequently Asked Questions

My EX Jazz still won’t start after touching the fob to the button. Confirm the brake is pressed fully and that you used the flat back of the fob over the button. Press START twice quickly while holding the fob against it.

How do I know if my Mk3 has Smart Entry or a turn-key? A red START/STOP button and no key slot means Smart Entry. A barrel you slot the key into means the conventional turn-key system — typically the S and SE grades.

Can the emergency key start the car? Only on turn-key grades. On Smart Entry cars the blade just unlocks the door — starting always goes through the push button.

Which battery does my Jazz fob take? Most take a CR2032; the thinner fob uses a CR1620. Flip the fob over or read the spent cell to confirm before buying.

Does a weak 12V battery cause the same warning? It can. If a fresh coin cell doesn’t restore keyless operation and the engine cranks slowly, suspect the 12V battery under the bonnet.

If a warning light stayed on after you got going, you can look up the exact fault code on our sister site autodtcs.com to see whether it relates to the smart-key system or something unrelated.