W212 Key Stopped working, but new battery hasn't fixed it?

[jjc2020]

This should be a simple question, but I can't find an answer. On my W212, the key stopped working. I've replaced the battery and it still doesn't work. The other key still works fine. With the new battery in, the broken key does light up when pressed but it has no effect on the car so it has power in the key, yet no longer seems to be paired to the car.

Are there any quick fixes, or is it just a new key needed?

Thanks

 

[Diesel Benz]

Do you mean you cannot start the car or open/close doors?

You do not need the battery when starting the car with the key in the ignition slot. If it does not work, it usually is a broken coil that powers the key in the EIS.

 

[jjc2020]

Do you mean you cannot start the car or open/close doors?

You do not need the battery when starting the car with the key in the ignition slot. If it does not work, it usually is a broken coil that powers the key in the EIS.

The key does nothing, to anything.

As I mentioned though, the other key works fine so I don't think it can be the EIS as that would affect all keys? Or am I missing something? Can it affect just the one key?

 

[jjc2020]

Here's ChatGPT's take. Seems like a fair interpretation?:

"Thanks — that helps narrow it down. Based on your description:

• Spare key works perfectly
• "Broken" key has battery power (light flashes)
• But broken key does not unlock car or start it, even in ignition slot

What This Means:​

Your "broken" key has power but is not being recognised by the car’s EIS, so:

1. The battery and buttons (remote lock/unlock) are functioning
2. But the infrared or transponder chip inside the key is probably damaged or corrupted
3. The EIS is working fine, because it recognises the spare key — so the issue is just this key

Likely Causes:​

• The transponder chip inside the key (used for ignition authentication) is damaged or disconnected
• The key may have been exposed to:
  • Moisture / impact / static
  • DIY battery change gone wrong (sometimes solder joints crack)
• IR transmitter in the key (for push-button unlock) may still function, but key-to-car handshake for starting is broken

What You Can Do:​

1. Key testing:
   • A Mercedes specialist with STAR diagnostics can plug into your car and confirm whether the broken key is sending any data to the EIS.
   • Some mobile key specialists can also test and clone the working key.
2. Repair or clone:
   • You can clone your working key to a new one for ~£150–£250 via an independent Mercedes key specialist.
   • Mercedes main dealers will want to replace the whole key (and possibly reprogram both), often >£300.
3. If you're techy:
   • Don’t try DIY repairs unless you're confident with soldering and chip work — the boards are delicate and EIS systems are encrypted.

In Summary:​

Your key has battery power but no authentication function. The internal chip is likely dead. Since your spare key works, the car is fine — and your best option is a new key cloned from the working one or a replacement via a specialist.

Would you like a shortlist of good UK Mercedes key repair specialists?

 

[ALFAitalia]

If it won't start the car or open the door that's wierd....two completely separate systems/faults happened at exactly the same time. To start the car you don't even need a battery in the key as the transponder chip does not use electricity. Short of the key being submerged in water for some time it's very unlikely for this to happen.

 

[jjc2020]

If it won't start the car or open the door that's wierd....two completely separate systems/faults happened at exactly the same time. To start the car you don't even need a battery in the key as the transponder chip does not use electricity. Short of the key being submerged in water for some time it's very unlikely for this to happen.

I hear you, but the evidence is clear. The key has lights, but it doesn't work doors or locks, and won't start the car and isn't even recognised - nothing happens.

But the spare key works fine.

So I'm kind of agreeing with ChatGPT - transponder chip died for some reason. Unless anyone else has another solution?

 

[ALFAitalia]

Transponder certainly sounds to be dead....but the transponder is not involved with remote locking.

 

[jjc2020]

Transponder certainly sounds to be dead....but the transponder is not involved with remote locking.

In which case I'm utterly confused. Can the overall code for the key and all its systems spontaneously change? Is that a thing? If so can it be recoded?

 

[OneForTheRoad]

In which case I'm utterly confused. Can the overall code for the key and all its systems spontaneously change? Is that a thing? If so can it be recoded?

Try pressing and holding lock and unlock together, until the red light flashes a few times.

 

[jjc2020]

Try pressing and holding lock and unlock together, until the red light flashes a few times.

I gave that a go, and yes held until it flashed a few times, but still no good. Thank you for the suggestion - from the behaviour it does seem like a reset and one that should have worked.

I think I'd best clone the still working one while I can! Maybe I dropped it or something in an awkward way. Thank you.

 

[ShaunB]

I gave that a go, and yes held until it flashed a few times, but still no good. Thank you for the suggestion - from the behaviour it does seem like a reset and one that should have worked.

I think I'd best clone the still working one while I can! Maybe I dropped it or something in an awkward way. Thank you.

I used these guys as very long time ago, like 10 years ago or something like that

Mercedes key specialists- Need new Mercedes keys? Replacementmercedeskeys.co.uk

Replacement key for your Mercedes. Lost, Spare or broken Mercedes keys we do them all! Call now - Car Key Specialists | Lost MERCEDES Keys and key Fob Experts - Quote: 07563014634 01217722808 - Nationwide service

www.replacementmercedeskeys.co.uk

From memory I sent the faulty key to them with guaranteed nextday 9am delivery via royal mail, they did whatever they did and send the key back to me the following day also 9am delivery and all was sorted.
Can't remember what they charged though to be fair, I just remember the man saying that they charged a flat fee regardless of what might be wrong with it.
My key cam back in a new case, I even think you have the option on upgrading the case to a newer version etc