Mercedes Infrared Key Won’t Turn: Key Fault or Ignition Switch (EIS) Fault?
Updated: Oct 9
Your Mercedes key goes into the ignition but won’t turn. It isn’t worn or damaged, and it looks perfectly fine. There are two main suspects: the electronics inside the key, or the Electronic Ignition Switch (EIS) behind the key slot. Both cause the same symptom, so it’s worth finding out which one you have before you pay for a part.
How the EIS system works
The EIS has a coil ring around the key slot. When you insert an infrared key, the ring powers the key by induction, and the key answers over infrared with a security code. If the EIS accepts the answer, it releases the ignition and allows the car to start.
That means the key has to be able to answer, and the EIS has to be able to power it, listen and decide. A fault on either side can leave the key stuck.
Does the key battery matter?
Usually not for this problem. The battery in the key runs the remote lock buttons. The ignition handshake is powered through the EIS, so a new battery typically won’t fix a key that won’t turn.
Why a Mercedes key that looks fine can still be faulty
The part of the key that answers the EIS is a chip and coil on the key’s circuit board. Workshop and owner reports describe keys where the chip stops sending a signal, leaving the ignition locked, and keys where the coil has come loose inside. None of this is visible from the outside.
The Mercedes spare key test
This is the quickest way to narrow it down:
The spare turns and the main key doesn’t: the electronics in the main key are the likely cause.
No key turns: the fault is more likely in the EIS, or less commonly the steering lock (ESL).
One caution: if both keys are the same age, both can fail. Also check the car’s own battery, because low voltage can stop the EIS from working.
Signs it points to the EIS
No key will turn, including the spare
The problem is intermittent, or worse in cold weather
The key turns but the dashboard stays dark, or the car won’t start
Common causes include cracked solder joints on the circuit board, moisture and corrosion around the key-reading coil, wear, and communication faults with the steering lock. Moisture in the footwell is a frequently cited problem on the E-Class W211 and the Sprinter. A flat battery or a jump start with the key in the ignition can also upset the EIS.
What happens next
If the EIS has failed, a replacement unit has to be matched to your car’s VIN, so it isn’t a simple swap. We repair the unit, and in many cases we can clone its data into a replacement so your existing keys keep working. See our EIS repair page.
If the key is at fault, see our Mercedes key page.
FAQ
Will a new key battery fix a key that won’t turn?
Usually not. The battery runs the remote buttons, while the ignition handshake is powered through the EIS.
My key looks perfect. Can it still be the problem?
Yes. The chip and coil inside can fail without any outward sign.
Can the EIS be repaired rather than replaced?
Often, yes. We can repair the unit or clone its data into a replacement, depending on the car.
Can I use my spare key?
If the spare turns, use it for now and have the main key checked. If it doesn’t, the fault is more likely in the car.
Why not just buy an EIS from eBay?
A replacement has to be coded to your car’s VIN, so it’s not a bolt-on swap. Eurotronix can assist with programming the EIS to your car.
Mercedes key won’t turn?
Call 0413 839 341 or send your model, year and a photo of the key.
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