These instructions apply to the Audi A4 Mk5 (B9/8W) 2015-Present.
For other models, please choose your vehicle here.
When the remote on your Audi A4 B9 (2015–present) starts working only from arm’s length, ignores the odd button press, or won’t lock the car at all, the cause is nearly always the coin cell inside the key running flat. The B9 — the MLB Evo-platform A4 saloon and Avant, shared closely with the A5, Q5 and S4 — uses Audi’s slim flat “smart” key with a pull-out emergency blade. Whether your car has the standard remote or the optional Advanced Key keyless system, the fob opens the same way and takes the same battery. Here is which cell to buy, how to open the case without cracking it, and why cutting a brand-new key is the one part of this job you can’t finish at home.
How to Tell the Fob Battery Is Dying
- Shrinking range. You have to stand right next to the A4 — or hold the fob against the door handle — before it locks or unlocks. This is the first and clearest sign.
- A dash warning. On Advanced Key cars the driver’s information display shows a key-battery-low message in the cluster when the fob voltage drops, often the first time you press Start with a weak key.
- Keyless entry gets fussy. The car stops recognising the key in your pocket, so the door handles won’t respond to a touch and you fall back on pressing the button.
- Cold-morning dropouts. A near-flat lithium cell sags in the cold, so the remote may work by afternoon but fail first thing.
A CR2032 in an A4 key typically lasts two to four years. A keyless Advanced Key fob tends to drain a little faster than a plain remote because it wakes up whenever it senses the car nearby. If yours is a few years old and showing any of the above, fit a fresh cell before you start suspecting the car.
The Battery You Need
The A4 B9 key takes a single CR2032 — a 3-volt lithium coin cell, 20 mm across — and it is the same cell whether you have the standard remote or the Advanced Key fob. It’s the most common coin cell there is, sold in any supermarket, hardware shop or petrol station. Buy a fresh, good-quality branded cell: cheap or long-shelved batteries sag under the brief high current the transmitter draws and can give poor range even when supposedly “new”. If you ever want to double-check, the code is printed on the flat face of the cell you remove — match it exactly.
How to Open the A4 B9 Key and Swap the Cell
- Slide out the emergency key blade. On the back of the fob there’s a small sliding catch. Push it across and pull the metal blade straight out of the housing. Keep it — you use it in a moment, and it’s also your way into the car if the fob is completely dead.
- Slide off the battery cover. With the blade gone, the small plastic cover on the back of the fob slides free from the slot the blade left behind. Ease it off with a fingernail, the emergency blade tip, or a small coin. Work slowly — the case is thin plastic and levering hard cracks it.
- Note the orientation, then lift out the old cell. Look at which way the battery sits before you disturb it. In the A4 fob the positive (+) face sits upward, toward the cover you just removed. Slide or gently prise the old cell out from under its retaining clip.
- Fit the new CR2032. Drop the fresh cell in the same way round — positive (+) side up — and press it gently until it seats under the clip against the contacts. Don’t force it; it should sit flat.
- Reassemble. Slide the cover back on until it clicks home, then push the emergency blade back into its channel until the catch latches. Test every button.
⚠️ Match the polarity you found. Fitting the cell upside down at best does nothing and at worst slowly discharges the new battery against the contacts. If you’re ever unsure, the rule never fails: copy the exact orientation of the cell you took out.
Check It Worked
Stand back a few metres and lock, then unlock, the A4. Full range should return straight away. On an Advanced Key car, walk up and try the keyless door-handle touch and the Start button — both should respond normally. Any key-battery-low message in the cluster usually clears the next time the car reads a healthy fob, occasionally after one drive rather than the instant you close the door.
If a Fresh Cell Doesn’t Fix It
- Re-check seating and polarity. The cell must sit flat under its clip, positive side up, pressed firmly onto clean contacts. A battery that’s slightly proud or upside down is the most common reason a “new” cell doesn’t help.
- Wipe the contacts. A smear of finger grease or a little corrosion on the terminals can cause dropouts — clean them with a dry cloth before you blame the fob.
- Try the backup start. If the car won’t recognise the key at all, hold the fob against the marked reader spot and press Start — if it starts that way, the chip is fine and only the battery side is weak.
- Rule out the car’s 12 V battery. A tired vehicle battery can make the whole car slow to wake and mimic key faults, so check it before condemning the key.
New Key or Spare — Why It’s a Dealer Job
Changing the cell keeps the fob’s existing coding, so nothing needs reprogramming and the key works the moment you close it up. A brand-new key is a different matter. The blade has to be cut to the car’s key code, and the fob’s transponder must be married to the A4’s immobiliser through the car’s security system — a process locked behind Audi’s online servers and dealer diagnostic access. That’s why adding a spare or replacing a lost key means a trip to an Audi dealer or a properly equipped auto-locksmith, and why it’s worth keeping the battery in your spare key fresh too, so you’re never down to a single working fob.
Frequently Asked Questions
What battery does an Audi A4 B9 key fob take?
A single CR2032 3-volt lithium coin cell. It’s the same cell for the standard remote and the Advanced Key keyless fob.
Which way up does the battery go?
Positive (+) side up, facing the cover you slide off. The surest guide is to note how the old cell sits and fit the new one exactly the same way.
Do I need to reprogram the key after changing the battery?
No. A battery swap doesn’t touch the coding or the immobiliser pairing, so the fob and keyless entry work straight away. Only a genuinely new key needs programming.
How do I get into the fob — is there a screw?
No screw. Slide out the emergency key blade first, then slide the small battery cover off the back where the blade came out. Everything is a slide-and-clip fit.
How long should an A4 key battery last?
Usually two to four years. An Advanced Key fob drains a bit quicker because it wakes whenever it senses the car, especially if it’s kept near the vehicle at home.
Can I still drive the A4 if the fob battery is completely dead?
Yes. Use the emergency blade hidden in the fob to unlock the driver’s door, then hold the fob against the marked reader spot and press the Start button — the car reads the transponder directly and starts, so a flat coin cell won’t strand you.
If the remote died suddenly right after the car’s 12 V battery was disconnected or went flat — rather than fading over weeks — the fault is more likely the car’s wake-up state than the coin cell, and it may have logged a fault. You can look up any stored code on autodtcs.com.
Independent guide — not affiliated with Audi. Always follow your official service manual and safety precautions. Read our full disclaimer.