These instructions apply to the Subaru Legacy Mk6 (BN) 2014-2019. For other models, please choose your vehicle here.
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The Subaru Legacy (Mk6/BN, 2014–2019) with Keyless Access and push-button start uses the START/STOP button as a back-up reader for a flat fob. Hold the access key fob against the button and the BN Legacy still recognises the chip, so it starts even when the coin cell is dead. Lower trims with a bladed turn-key ignition start as normal.
Start the BN Legacy with a dead fob
- Unlock and get in with the emergency key first (next section), then sit in the driver’s seat and close the door.
- Press and hold the brake pedal.
- Hold the access key fob directly against the push-button START/STOP switch and keep it pressed there — the reader coil sits behind the button.
- With the fob still held to the button, press the button. The chip wakes at close range and the engine starts.
If the meter shows the key is not detected, hold the fob flat and square against the button and try again — the read range is only about a centimetre.
Unlock the BN Legacy with the emergency key
- Slide the small latch on the back of the access key fob and pull out the metal emergency key.
- At the driver’s door handle, insert the key into the lock cylinder — it may sit beneath a slim cover you pop off first.
- Turn the key to unlock. The alarm may chirp until the engine starts and the transponder is read.
Immobiliser note
The fob’s coin cell only powers the proximity broadcast and remote buttons. Engine authorisation is handled by a passive transponder energised by the start-button coil, so a flat cell never stops the Legacy starting — it only disables hands-free locking and the remote. On bladed-key trims a dead fob battery does not affect starting at all.
Replace the fob battery
Most BN Legacy access keys (2016-on) use one CR2032 3V lithium coin cell in a screw-less snap-fit case. Pull the emergency key, prise the halves apart at the key-slot seam, swap the cell + side up, and click it shut. Very early 2014–2015 fobs may instead use a CR2025 secured by a small Phillips screw — check the printing on the old cell before buying. No re-coding is needed after a swap.
Frequently Asked Questions
Is my Legacy fob a CR2032 or a CR2025?
Screw-less snap-together fobs (mostly 2016-on) take a CR2032; early screw-fastened fobs take a CR2025. The number is printed on the old cell — match it exactly.
Do I hold the fob to the start button or somewhere else?
To the START/STOP button. On the Mk6 Legacy the back-up reader is built into the button, so that is where the dead fob must touch.
My Legacy has a turn-key ignition — does this matter?
No. With a bladed ignition you just unlock and turn the key; a dead fob battery has no effect on starting. The dead-fob trick is only for Keyless Access push-button cars.
Is it the fob or the car’s 12V battery?
If the cabin stays dark when you open the door, suspect the 12V battery, not the coin cell. A flat coin cell only affects the remote and hands-free locking.
Will swapping the cell erase my fob pairing?
No. The transponder and pairing are unaffected; remote and keyless functions return immediately after the battery swap.
If a warning light stayed on after the no-start, look it up on autodtcs.com to confirm it was only the fob and not a stored 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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