Start the Car With a Dead Key Fob Battery

Last updated: June 12, 2026

The Subaru Outback (Mk5/BS, 2015–2019) with Keyless Access and push-button start uses the START/STOP button as a back-up reader when the access key fob’s battery goes flat. Hold the fob against the button and the immobiliser still energises the chip, so the BS Outback starts even with a dead coin cell.

Start the BS Outback with a dead fob

1. Unlock and climb in with the emergency key first (next section), then sit in the driver’s seat and close the door.
2. Press and hold the brake pedal firmly.
3. Touch the access key fob directly against the push-button START/STOP switch on the dash and keep it pressed there — the reader coil sits behind the button.
4. 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 recognised, hold the fob flat and square against the button and try again — effective range is only about a centimetre.

Unlock the BS Outback with the emergency key

1. Slide the release catch on the back of the access key fob and draw out the metal emergency key.
2. At the driver’s door, insert the key into the lock cylinder on the handle and turn to unlock.
3. The alarm may sound until the engine starts and the system reads the transponder.

One caveat owners report on the BS: the door lock is electrically assisted, so if the car’s 12V battery is also completely flat the mechanical key may not throw the lock. In that case the 12V battery, not the fob, is the real problem and needs a jump or charge first.

Immobiliser note

The fob’s coin cell only runs the proximity signal and remote buttons. Engine authorisation comes from a passive transponder powered inductively by the start-button coil, so a dead cell never blocks starting — it just disables hands-free locking and the remote.

Replace the fob battery

Most BS Outback access keys use one CR2032 3V lithium coin cell in a snap-fit case. Pull the emergency key, prise the halves apart at the key-slot seam, swap the cell + side up, and click it back together. The earliest fobs may instead use a CR2025 with a small screw — check the old cell’s printing before buying. No re-coding is needed after a swap.

Frequently Asked Questions

The mechanical key won’t unlock my BS Outback — why?

On this generation the door lock is power-assisted. If the car’s 12V battery is flat the blade can fail to turn the lock. Sort the 12V battery first; the dead-fob issue is separate.

Do I hold the fob to the start button or somewhere else?

To the START/STOP button. The Mk5 Outback’s back-up reader is built into the button, so that is where the dead fob must touch.

Is my fob a CR2032 or CR2025?

Later 2015–2019 fobs use a screw-less CR2032; some very early ones use a CR2025 held by a small screw. The number is printed on the old cell — match it.

Is it the fob or the car battery?

If nothing lights up when you open the door, suspect the 12V battery. A flat coin cell only affects the remote and hands-free locking, not the cabin.

Will replacing the coin cell wipe my settings?

No. The transponder and pairing are unaffected; remote and keyless functions return immediately after the 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 fault code.