These instructions apply to the Mercedes A-Class Mk4 (W177) 2018-Present. For other models, please choose your vehicle here.
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If the SmartKey on your Mercedes A-Class Mk4 (W177, 2018–present) is getting weak — short range, fussy keyless entry (KEYLESS-GO), or a “key battery” message — the coin cell needs renewing. The A-Class key takes a CR2032 and opens with the emergency key blade. No programming needed.
What You’ll Need
- A fresh CR2032 3 V lithium coin cell.
- The emergency key blade (slides out of the fob).
How to Replace the A-Class W177 Key Battery
- Slide out the emergency key blade — push the release catch on the back of the fob and pull the blade out.
- With the blade removed, use the slot to slide off the back cover of the fob (it slides in the direction the blade came out).
- Lift out the old CR2032, noting which way up it sits.
- Fit the new cell the same way, by the edges.
- Slide the cover back on and refit the blade.
- Test lock/unlock from a normal distance.
Does It Need Programming?
No — the swap doesn’t affect the key’s pairing, so it works straight away. A lost or extra SmartKey can only be ordered and coded by a Mercedes dealer (with proof of ownership).
If It Still Won’t Respond
Re-open the fob, check the cell orientation and clean the contacts. If KEYLESS-GO is slow, hold the key against the start button / marked spot to start, then it re-syncs.
Frequently Asked Questions
What battery does the A-Class W177 key take?
A single CR2032 3 V lithium coin cell.
Will the car start with a flat key battery?
Yes — hold the SmartKey against the start button (or the marked spot) to start on the backup transponder; the emergency blade unlocks the driver’s door.
Which way up does the cell go?
Match the orientation of the cell you removed.
Can I program a spare key myself?
No — Mercedes key coding needs the dealer system and proof of ownership. Only the battery swap is DIY.
To clear a service reminder after maintenance, see our A-Class W177 service reset guide. For warning lights, look up the code on autodtcs.com.
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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