These instructions apply to the Audi A4 Mk5 (B9/8W) 2015-Present. For other models, please choose your vehicle here.
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The Audi A4 (B9, 2015–Present) is a push-button-start car, and like the rest of the current Audi range it uses the start button itself as the back-up reader for a dead fob. Hold the flat remote against the START ENGINE STOP button and its inductive coil wakes the transponder, so the A4 starts even with a completely flat coin cell.
Start the A4 B9 with a dead fob
- Open the car with the emergency blade first (next section), then sit in the driver’s seat and close the door.
- Press and hold the brake pedal.
- Hold the flat face of the fob directly against the START ENGINE STOP button on the centre console — the button houses the back-up reader coil.
- Keeping the fob pressed there, push the START button. The chip is energised inductively and the engine cranks.
If the dash shows the key is not detected, press the fob flat and square against the button and try again — the read range is only a centimetre or so.
Unlock the A4 B9 with the emergency key blade
- On the back or bottom edge of the fob, press or slide the release and pull the metal blade out.
- At the driver’s door, the lock barrel hides under a small cover on the handle. Use the blade or a fingernail to pop that cover off.
- Insert the blade into the barrel and turn to unlock. The alarm may chirp until the engine starts.
Why the A4 starts with a dead fob
The fob’s coin cell powers only the keyless proximity broadcast and the remote-lock buttons. Engine authorisation uses a passive RFID transponder that takes its power inductively from the coil behind the start button — not from the cell. That is why pressing the dead fob against the button lets the immobiliser read the chip and start the car.
Replace the fob battery
The A4 B9 fob uses one CR2032 3V lithium coin cell. Slide out the emergency blade, then squeeze the release clips inside the key slot to draw the battery tray straight out — do not prise the slim casing. Fit the new cell + side up, refit the tray and the blade, and remote and keyless functions return at once with no coding.
Frequently Asked Questions
Do I hold the fob to the steering column or the start button?
To the START button. On the B9, the inductive back-up reader is behind the start button, so that is where the dead fob must touch.
Some A4 B9s have a slot near the start button — what’s that?
Earlier B9s offered a fob holder/slot. If yours has one, you can insert the fob into the slot and then press to start with your foot on the brake; otherwise hold the fob flat against the button.
How do I open the fob without breaking it?
Don’t prise the casing. Pull the blade, then squeeze the clips inside the key slot to slide the battery tray straight out.
Why does the alarm sound when I unlock with the blade?
The car treats a mechanical unlock as forced entry until it reads the transponder. The alarm stops the moment the engine starts.
Is it the fob or the 12V battery?
If the cockpit is dead and nothing wakes when you open the door, suspect the car’s 12V battery, not the coin cell. A weak cell only affects the remote.
If a warning light stayed on after the no-start, look it up on autodtcs.com to confirm it was just the fob rather than an immobiliser code.
Disclaimer: The information on this page is provided for general guidance only. Always follow your official service manual and safety precautions when working on your vehicle. We are not responsible for errors, omissions, or any damage resulting from the use of this information.
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