These instructions apply to the BMW 1 Series Mk3 (F40) 2019-Present.
For other models, please choose your vehicle here.
The BMW 1 Series Mk3 (F40, 2019–present) — the front-wheel-drive generation that replaced the old rear-drive hatchback — uses a conventional keyless-comfort fob, not one of BMW’s rechargeable “Display Keys.” That’s good news: when the remote starts working only up close, or the car flashes a “Key battery low” message, you can swap the coin cell yourself in a couple of minutes. The F40 fob opens in an unusual way — you use the slide-out valet blade itself as a tool — and it’s worth knowing that trick before you start prying. Here is the exact battery, the correct opening method, and why a brand-new key still has to go through BMW.
How to Tell the Fob Battery Is Dying
- Shrinking range. You have to stand next to the car, or hold the fob to the door handle, for it to lock or unlock — usually the first thing you notice.
- Comfort Access gets unreliable. Walk-up locking and the handle-touch unlock stop recognising the fob in your pocket, so you fall back to pressing the buttons.
- A “Key battery low” message. The F40’s instrument display shows a low-key-battery warning when the fob voltage drops — your clearest early cue.
- Cold-morning dropouts. A near-flat cell that’s fine in the afternoon can fail on a frosty morning; intermittent cold-weather response points to the battery.
A coin cell in an F40 fob typically lasts two to four years. BMW’s Comfort Access is fairly sensitive to voltage, so the warning can appear before the cell is truly flat — if the fob is a couple of years old and playing up, a fresh battery is the sensible first move.
The Battery You Need
The BMW 1 Series F40 fob takes a CR2032 3-volt lithium coin cell — the common, widely stocked size. This matters because BMW’s system is picky about voltage: cheap or old-stock cells often trigger the “Replace battery” message even when they read as new, so fit a good branded cell (Panasonic, Sony/Murata, Energizer or Duracell). If your car came with the rechargeable touchscreen Display Key as an extra, that one charges in the car or on a Qi pad and has no user-replaceable cell — but the standard F40 fob in most owners’ hands is the conventional CR2032 type described here.
How to Change the F40 Fob Battery
- Release the valet blade. Press the small button/catch on the side or back of the fob and pull the metal emergency key straight out. This is both your access tool and what unlocks the cover.
- Locate the cover release in the blade channel. With the blade removed, look into the slot it came from — on the BMW fob the cover is released from inside that channel, not by prying the outer seam.
- Use the blade to release the cover. Insert the tip of the valet blade (or a small flat tool) into the slot and lever/turn gently to release the battery cover, then slide or lift the back cover off. Take it steadily — the cover is designed to come away with light pressure.
- Note the orientation, then remove the old cell. Look at which face of the coin cell shows before you lift it out. Ease the old CR2032 from under its retaining clip.
- Fit the fresh CR2032. Insert the new cell the same way round as the old one, following the +/− markings by the holder, and make sure it’s fully seated. Handle it by the edges to keep grease off the faces.
- Refit the cover and blade. Press the cover back until it clicks home flush, then slide the valet blade back in until it latches.
⚠️ Mind the polarity. A coin cell fitted the wrong way up won’t power the fob and can slowly discharge if left in place. Match the orientation of the cell you removed, and follow the + symbol moulded into the holder — never fit it blind.
Confirm It Worked
Stand back a few metres and lock, then unlock, the car with the buttons — full range should return at once. Then test Comfort Access: with the fob in your pocket, touch the door handle to lock/unlock and press start to confirm the car reads it. The “Key battery low” message clears once the car next reads a healthy fob, which can be after a drive cycle rather than immediately.
If a Fresh Cell Doesn’t Fix It
- Use a good-quality cell. This is the number-one fix on BMWs: a weak supermarket or old-stock CR2032 can throw the warning straight away. Swap in a fresh branded battery.
- Re-check polarity and seating. Make sure the cell is the right way up and pressed fully under its clip against clean contacts.
- Clean the contacts. A dry cloth or cotton bud on the terminals removes grease or light corrosion that causes dropouts.
- Try the backup start. If the car won’t detect the fob, hold it against the marked spot near the steering column/start button and press start — BMW’s emergency read works at close range even with a weak cell, confirming the battery was the issue. A tired car 12 V battery can also mimic key faults, so have it tested if the whole car is slow to wake.
New Key or Spare — Why It’s a Dealer Job
Changing the coin cell leaves the fob’s coding untouched, so no reprogramming is needed — the key works the moment the cover clicks shut. A genuinely new key is different: BMW keys are tied to the car’s CAS/immobiliser and are order-restricted — a replacement is supplied against the VIN with proof of ownership and then enabled for your specific car. That process runs through a BMW dealer (or a specialist with BMW-level diagnostics), so a spare or a lost-key replacement isn’t a driveway job. Keep your spare fob’s battery fresh too, and you’ll never be down to a single working key.
Frequently Asked Questions
What battery does a BMW 1 Series F40 key fob take?
A single CR2032 3-volt lithium coin cell — the standard, easily found size. Use a quality branded one, as BMW’s system is sensitive to low voltage.
How do I open the F40 fob?
Pull out the valet blade, then use the blade (or a small flat tool) in the slot it left to release the battery cover — you don’t pry the outer seam. The cover then lifts or slides off.
Which way up does the battery go?
Match the cell you removed and follow the +/− markings by the holder. Note the orientation before lifting the old one out so you can’t get it wrong.
Do I need to reprogram the key after a battery change?
No. The coding isn’t affected by swapping the cell, so the fob works immediately. Only a brand-new key needs enabling for the car.
Why does my BMW still say “Replace battery” after a new cell?
Usually because the new cell is weak or old stock — Comfort Access is fussy about voltage. Fit a fresh, good-quality branded CR2032 and re-check the polarity and seating; the message clears once the car reads a healthy fob.
Can I still start the car if the fob is dead?
Yes. Use the valet blade to unlock the driver’s door, then hold the fob against the emergency-read spot by the column and press start — the car reads the transponder up close, so a flat cell won’t strand you.
If your F40’s remote failed suddenly rather than fading — especially just after the 12 V battery was disconnected or gone flat — suspect an electrical or pairing issue over the cell, and any stored fault code will help you pin it down. You can look up what a dashboard code means on autodtcs.com.
Independent guide — not affiliated with BMW. Always follow your official service manual and safety precautions. Read our full disclaimer.