Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

When the remote on your BMW 1 Series (F40, 2019–Present) stops unlocking the car or the cluster shows a key-battery warning, the coin cell inside the fob has gone flat. The F40 – BMW’s first front-wheel-drive 1 Series – has no dashboard key slot, so the fix is to unlock with the hidden blade and start the engine by holding the fob against a reader in the steering column. Neither step needs any tools beyond the fob itself.

Unlock the door with the emergency key blade

A flat fob cannot trigger the central locking, so begin at the driver’s door with the metal blade hidden in the remote.

1. Slide the release catch on the base of the fob and pull the metal emergency blade out of the housing.
2. At the driver’s door, note there is no exposed keyhole – the lock barrel sits under a trim cap on the door handle.
3. Insert the blade into the recess beneath the handle and lever the cap off to expose the lock cylinder.
4. Turn the blade to unlock the driver’s door, then refit the cap once you are inside.

Unlocking this way may set off the alarm; it stops as soon as the car recognises a valid key in the cabin.

Start the engine by holding the fob to the steering column

The F40 reads the fob’s transponder through a ring antenna in the steering-column surround.

1. Sit in the driver’s seat, close the door, and press the brake pedal.
2. Look for the small key symbol embossed on the steering-column trim, usually on the right-hand side.
3. Press the flat back of the fob firmly against that symbol so it makes solid contact with the plastic.
4. With the fob held in place and the brake down, press the Start/Stop button to start the engine.
5. If nothing happens, reposition the fob – pressing a different face or the blade end against the symbol can help the antenna find the coil.

Why the chip starts the car and the blade does not

Only the remote functions – locking, tailgate, Comfort Access – rely on the coin cell. Engine authorisation comes from a passive transponder chip that needs no battery of its own; the column antenna energises it and reads its security code to release the immobiliser. The blade is purely mechanical, so it opens the door but cannot start the car. This is the same on every keyless F40.

Replace the fob battery (CR2032)

Once you are moving again, fit a fresh cell to restore normal keyless use.

1. Remove the emergency blade; the gap it leaves is the leverage point for opening the case.
2. Ease the two halves of the housing apart along the seam until the back cover releases.
3. Lift out the old CR2032 coin cell, noting its orientation, and press the new one in to match.
4. Snap the case shut, refit the blade, and lock/unlock the car to confirm the remote works again.

If the warning persists after a new cell, clean the contacts with a dry cotton bud, as light corrosion can mimic a weak battery.

Frequently Asked Questions

Does the F40 have a key slot? No. The F40 uses only the steering-column reader, so there is no slot to insert the fob – hold it against the key symbol on the column.

Where is the key symbol on the F40? On the plastic surround around the steering column, generally on the right-hand face. It is a small embossed key outline.

The alarm sounded when I unlocked with the blade – is that wrong? No. Mechanical unlocking bypasses the remote, so the alarm arms briefly and silences once the immobiliser reads your key inside.

Held the fob to the column and it still won’t start – why? Reposition and retry first. If the car is dead or only clicks, suspect the F40’s 12 V battery rather than the fob.

What battery does the F40 fob use? A single CR2032 3 V lithium coin cell. Handle it by the edges to preserve its life.

If a warning light stayed on after the key trouble, you can look up what it means on our sister site autodtcs.com.