These instructions apply to the BMW 3 Series Mk7 (G20) 2019-Present.
For other models, please choose your vehicle here.
If the remote on your BMW 3 Series Mk7 (G20, 2019–present) is getting weak — you have to stand right by the car to lock it, the buttons only work on the second or third press, Comfort Access is fussy, or a “key battery low” message shows in the cluster — the coin cell inside the fob has simply worn out. It is not a fault with the immobiliser, the doors or the car’s electronics: it is the small 3 V battery that powers the remote. The G20 key takes a CR2032 and opens with the emergency blade hidden inside, and because the fob only carries its coding rather than storing it, swapping the cell needs no programming at all. It is a two-minute job at the kitchen table.
How to Tell the Fob Battery Is Dying
The cell fades gradually, so the warning signs build up over a few weeks rather than appearing all at once:
- Shrinking range. You once locked the car from across the car park; now it only answers within a metre or two.
- A “key battery low” message in the instrument cluster — the car monitors the fob and flags a low cell in good time.
- Comfort Access only works up close. A proximity key transmits constantly to answer the door handle and start button, so it drains faster than a button-only remote and is the first function to weaken.
- Second or third press needed to lock or unlock, or the fob working only intermittently.
- Worse in cold weather — a marginal coin cell struggles on frosty mornings and seems to recover once it warms up.
As a rough guide a button-only remote runs two to four years on one cell, while a Comfort Access fob that is always awake is often nearer a year. Leaving the fob next to another transmitter or charger overnight can shorten that.
The Battery You Need
The standard G20 fob uses a single CR2032 — a 20 mm, 3 V lithium coin cell sold in every supermarket, hardware shop and pharmacy. Buy a branded cell (Panasonic, Sony/Murata, Duracell, Energizer); the fresh voltage and shelf life are more dependable than cheap multipacks that can arrive half-flat. Do not fit a thinner CR2025 — it can sit loose, lose contact and give you the exact weak-range symptom you are trying to cure. If you are unsure, the code is stamped on the flat face of the old cell: take it out, read it and match it. For the standard 3 Series fob that code is CR2032.
How to Replace the G20 Key Battery
- Slide out the emergency key blade. Push the small catch on the fob and pull the metal blade out — this is the key you use to unlock the door manually if the fob ever dies, and its slot is the opening point for the case.
- Open the cover. With the blade removed, slide or gently lever the cover off the back of the fob, using the blade slot as the starting point. Ease it around the joint rather than forcing one corner.
- Note the orientation before you lift. Look at which way up the old CR2032 sits so the new one goes back exactly the same way.
- Lift out the old cell and fit the new one the same way round, by the edges only — skin oils on the flat faces leave a film that can weaken the contact.
- Click the cover back on until it seats, then slide the blade home until it clicks.
- Test lock and unlock, and Comfort Access, from a normal distance.
(The optional “Display Key”, if your car has one, is rechargeable via USB or inductive charging rather than a coin cell — it has no user-replaceable battery. If it stops responding, charge it rather than opening it.)
⚠️ Watch the polarity. Fitting the cell upside down is the most common reason a fob stays dead after a “new” battery. Match the way up of the cell you removed — note it before you lift the old one out and there is nothing to guess.
Make Sure It Took
Stand back a few metres and press lock, then unlock — the indicators should flash and the mirrors respond as usual. Try Comfort Access: with the fob in your pocket, touch the door handle to lock and unlock. A “key battery low” message usually clears itself after the next drive once the fob is transmitting at full strength again.
If a Fresh Cell Doesn’t Fix It
- Re-check polarity and seating. Re-open the fob and confirm the cell is the right way up and sitting flat under its clip.
- Confirm the code. Make sure you fitted a CR2032, not a thinner look-alike.
- Wipe the contacts. Clean the spring terminals and both faces of the cell with a dry cloth — a fingerprint film or light corrosion is enough to cause dropouts.
- Use the backup start. If Comfort Access is slow, hold the key against the start button (or the marked spot) to start — this uses the built-in transponder and re-syncs the remote.
- Rule out the 12 V car battery. A tired vehicle battery causes its own keyless quirks; if the car is also slow to wake or crank, have the 12 V checked.
New Key or Spare — Why It’s a Dealer Job
Changing the battery leaves the fob’s coding intact, which is why it works instantly with no programming. A lost or additional key is different: it must be coded to your 3 Series’ VIN with diagnostic equipment, so a new key can only be ordered and paired by a BMW dealer, with proof of ownership. If you keep a spare key in a drawer, change its cell every couple of years too — a flat spare is no help in an emergency.
Frequently Asked Questions
What battery does the 3 Series G20 key take?
The standard fob uses a single CR2032 — a 3 V lithium coin cell, 20 mm across. To be sure, read the code stamped on the old cell as you take it out. The optional Display Key is rechargeable and has no replaceable cell.
Which way up does the cell go?
The same way up as the cell you removed. Note the orientation before you lift the old one out. Fitting it upside down is the usual reason a fob stays dead after a fresh battery, so check before you click the cover back on.
Do I need to reprogram the key after changing the battery?
No. The battery only powers the remote; it does not store the pairing, so the key works the instant the new cell is in. Programming is only needed for a brand-new or replacement key, which is a dealer job.
How long should the new battery last?
A button-only remote manages two to four years per cell. With Comfort Access the fob is always awake and drains faster — expect nearer a year to eighteen months. Cold weather and storing the fob near other transmitters shorten it.
The key still won’t work after a new battery — what now?
Re-open the fob and confirm the CR2032 is seated the right way up on clean contacts. Wipe the terminals. If it still will not respond, use the backup start (hold the key to the start button/marked spot) and have the fob checked; also rule out a weak 12 V car battery.
Can I drive the 3 Series with a completely dead fob battery?
Yes. Use the emergency blade to unlock the driver’s door, get in, then hold the key against the start button (or the marked spot) to start on the built-in transponder. That gets you moving so you can fit a fresh CR2032 when it suits you.
To clear a service reminder after maintenance, see our 3 Series G20 service reset guide. For warning lights, look up the code on autodtcs.com.
Independent guide — not affiliated with BMW. Always follow your official service manual and safety precautions. Read our full disclaimer.