These instructions apply to the Smart Fortwo Mk2 (W451) 2007-2015. For other models, please choose your vehicle here.
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If the key fob on your Smart Fortwo (W451, 2007–2015) has gone weak or the locking has become intermittent, the coin cell inside is almost certainly flat. It’s a quick swap.
Battery First — What You Need
- A CR2032 coin cell (check the marking on the old one when you open the fob).
- A coin to open the case.
Replacing the Cell
- Use a coin to remove the button-cell cover.
- Separate the cases.
- Remove the old cell (don’t touch the terminals).
- Fit the new cell, observing the correct polarity.
- Refit the cases and check the remote locks and unlocks the car.
Lost Key or Adding a Spare
A cell swap keeps the existing coding. A lost or broken key can only be ordered and programmed by a smart/Mercedes dealer — there’s no owner programming sequence.
If a Fresh Cell Doesn’t Help
- Check the polarity — an upside-down cell won’t work.
- Hold the fob close to the car while the receiver re-syncs.
- Still intermittent? A receiver fault can store a code — look it up on autodtcs.com.
Frequently Asked Questions
What battery does the Smart Fortwo W451 key use?
A single coin cell, typically a CR2032 — confirm against the old one.
Do I need to reprogram the key after the swap?
No — the coding is retained for a battery change.
Can I program a new key myself?
No — a replacement must be ordered and coded by a dealer.
How do I open the fob?
Use a coin to twist off the cover, then separate the two halves.
If the fob stopped working after a battery disconnection, see our Smart Fortwo W451 battery disconnect/reconnect guide.
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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