Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

The Mercedes GLC (X253, 2015–2022) — the mid-size SUV that replaced the GLK — spans the slot-key and full KEYLESS-GO push-button eras, so what you do with a dead SmartKey depends on how your car was specified. Either way the immobiliser reads the key through a battery-free transponder, and a hidden metal blade gets you into the cabin. This guide covers both ignition styles plus the coin-cell swap.

Start the GLC with a dead key fob battery

The transponder inside the SmartKey is powered by the car, not its coin cell, so it still authorises a start when the fob battery is flat.

1. Get in, close the door, and press the brake pedal firmly.
2. If your GLC has the conventional ignition slot beside the steering column, push the SmartKey fully home and turn — the slot is the antenna and works with a dead battery.
3. If your car has KEYLESS-GO with a push-button START/STOP switch, hold the SmartKey flat against that button so the fob touches it, then press the button through the key.
4. The cluster shows the key symbol, then the engine cranks.
5. Replace the coin cell soon afterwards.

Unlock the door with the emergency key blade

If the doors will not unlock electronically, use the mechanical key hidden in the fob.

1. Slide the catch on the SmartKey and pull the metal emergency blade out.
2. The driver’s door keyhole is hidden under a cover cap on the door handle. Insert the blade tip into the recess at the rear of the handle and lever the cap off to reveal the barrel.
3. Insert the blade and turn to unlock. The alarm may sound — that is expected.
4. Get in and start with the method above; the alarm stops once the SmartKey is sensed. Refit the blade.

Why the immobiliser still trusts the key

The X253’s electronic ignition switch energises an induction coil at the start button or ignition slot. Held in that field, the SmartKey transponder powers up and sends its encrypted rolling code, authorising the start with no fob-battery involvement. The remote lock/unlock buttons need the coin cell; the start handshake does not.

Replace the key fob battery

The X253 chrome arrow-style SmartKey commonly uses two CR2025 coin cells stacked inside; always check the count and size you remove.

1. Slide out the emergency blade to release the cover.
2. Use the slot the blade left to prise the back cover off.
3. Note the orientation, then lift out the cells.
4. Fit fresh CR2025 cells the same way up, refit the cover and reinsert the blade.
5. Test the remote from a few metres — it should respond instantly.

Frequently Asked Questions

Does my X253 GLC have a slot or a push button? Both were sold across 2015–2022. Slot cars take the key directly; KEYLESS-GO cars use a push button you hold the fob against.

How many batteries does the GLC fob use? The X253 SmartKey commonly takes two CR2025 cells. Open yours to confirm before buying.

Where is the keyhole on the GLC’s door? Under a cap on the driver’s door handle. Lever the cap off and the barrel is underneath.

The alarm went off using the metal key — normal? Yes. Mechanical entry trips the alarm until the SmartKey is recognised inside; it then silences itself.

New battery, but the remote still does nothing. Re-seat the cells and check polarity. If the remote stays dead the fob may need resyncing, but the car still starts by holding the key to the button or using the slot.

If a warning lamp stayed lit after a flat key or weak 12V battery, decode the fault on our sister site autodtcs.com.