These instructions apply to the Mercedes C-Class Mk3 (W204) 2007-2014.
For other models, please choose your vehicle here.
Short answer: when the SmartKey on your Mercedes C-Class W204 (2007–2014) gets weak, slide out the emergency key blade, open the fob and swap the CR2025 3V coin cell (some keys take two). No coding is needed — it works straight away.
When the Key Needs a New Battery
A low SmartKey battery shows as a key/battery message and shorter range — you have to be closer to lock or unlock, and keyless entry becomes unreliable. Replacing the coin cell restores it; the key does not need re-programming afterwards.
Replace the SmartKey Battery
- Slide out the emergency key blade — press the release and pull the metal key out of the fob.
- With the blade removed, open the battery compartment (the cover releases where the blade sat, or the fob halves separate).
- Fit a fresh CR2025 3V cell the correct way round (if your key holds two, replace both with the same type).
- Clip the fob shut and slide the emergency key back in. Test lock/unlock.
Use the same battery type as the original; a different cell can give poor range or damage the key.
If the Key Still Won’t Work
If range is still poor with a fresh cell, check it’s seated and the right way round. If the car won’t detect the key at all, you can still start the car using the emergency start method (hold the key to the start button/slot) — a persistently dead key then needs a Mercedes dealer, who can read and register keys.
Frequently Asked Questions
What battery does a Mercedes W204 key take?
A CR2025 3V lithium coin cell. Some SmartKeys hold two — if so, replace both with the same type.
Do I need to reprogram the key after changing the battery?
No. Fit the new cell and the key works immediately — no coding required.
How do I open the W204 SmartKey?
Slide out the emergency key blade first; the battery compartment then opens where the blade sat. Don’t force the halves before removing the blade.
My key still has short range with a new battery — why?
Check the cell is the correct type and seated properly. If it’s still weak, the key electronics may be failing and need a dealer.
If a key or immobiliser warning showed on the dash, you can check what it means on our sister site autodtcs.com.
Independent guide — not affiliated with Mercedes. Always follow your official service manual and safety precautions. Read our full disclaimer.