Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

The Mercedes C-Class (W205, 2014–2023) spans both the slot-key and full KEYLESS-GO push-button eras, so what you do with a dead SmartKey depends on how your saloon, estate, coupe or cabriolet was specified. In every case the immobiliser reads the key through a battery-free transponder, and a metal emergency blade gets you into the cabin. This guide covers both ignition styles plus the coin-cell change.

Start the W205 with a dead key fob battery

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

1. Sit in, close the door, and press the brake pedal firmly.
2. If your W205 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. On the coupe and cabriolet it is more discreet, tucked under the handle; on saloon and estate it sits at the rear of the handle. Lever the cap off with the blade tip to expose 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 W205’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 at all. The remote lock/unlock buttons need the coin cell; the start handshake does not.

Replace the key fob battery

The W205 chrome arrow-style SmartKey typically 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 two CR2025 cells.
4. Fit two fresh CR2025s 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 W205 have a slot or a push button? Both were offered across the 2014–2023 run. Slot cars take the key directly; KEYLESS-GO cars use a push button you hold the fob against.

How many batteries does the C-Class fob use? The W205 SmartKey commonly takes two CR2025 cells. Open yours to confirm before buying.

Where is the keyhole on a C-Class coupe? Under the door handle rather than on its face — it is deliberately hidden. Pop the cap 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 both 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.