My Key Won’t Turn Ignition Mercedes-Benz Problem

Last updated: October 29, 2026

You insert the key into the ignition of your Mercedes-Benz, but nothing happens. You can’t turn the key. You can’t start the car. A low or discharged vehicle battery is one possible cause, but a key that will not turn can also result from a steering-lock problem, EIS/EZS fault, key authentication problem, or a mechanical ignition-lock/tumbler fault, depending on the vehicle. Before any of that, rule out the cheapest thing: change the battery in the key, and if the remote buttons are dead too, see a Mercedes key fob that is not working. As you insert the Smart Key remote into the Electronic Ignition Switch (EIS) control module (N73), power is supplied to the key by the induction coil ring located in the EIS. Data is then exchanged with the infrared transmitter to validate the Drive Authorization. This article discusses why the key won’t turn in the ignition and a possible solution.

Mercedes EIS / EZS Immobilizer Key Module Problems

List of common problems and possible solutions

• Locked Steering Wheel

• Discharged Battery and Undervoltage

The battery in the car may be defective, as it does not provide the necessary voltage for the ECU and the N73 Electronic Ignition/Starter Switch (EIS) to function correctly. If the battery is dead, has low voltage, or is undercharged, the CAN communication between EIS and N72 may not function properly. The EIS in the Mercedes-Benz has a coil that induces a magnetic field on the Smartkey, which causes the key to respond to the car with a specific rolling code. Using a 10-dollar multimeter like this, you can check your battery before proceeding. Measure the battery after the vehicle has been sitting without charging or recently running. A fully charged conventional 12-volt battery will typically measure around 12.6 to 12.8 V at rest. A significantly lower reading may indicate a discharged or weak battery. With the engine running, verify that the charging system is operating within the vehicle manufacturer’s specified range. If the battery fails testing, replace it with a battery that matches the vehicle manufacturer’s specified capacity, dimensions, terminal configuration, and battery technology. If you need a replacement battery for your Mercedes-Benz, the most cost-effective option would be to order one on Amazon. Most Mercedes-Benz cars use Battery Group 94R, and a great choice would be the ACDelco 94AGM Battery, which meets the OEM specifications. Click here to read reviews and check the price on Amazon. Battery size and specifications vary by Mercedes-Benz model, engine, production year, and equipment. Verify the correct replacement battery by VIN or the original battery specification.

• Key Tumbler Defective

• Defective Electronic Ignition/Switch System EIS / N73

• Smart Key Remote

• Defective Electronic Control Module (ECU)

Also, consider the following:

• I cleaned the EIS keyhole with a dry cloth. Do not use liquid to clean the EIS.
• Check the fuses and power/ground circuits associated with the EIS/EZS, steering-lock system, and relevant control modules using the correct vehicle-specific fuse chart.
• If available, try the vehicle’s spare key. If the spare key works normally, the original key becomes the primary suspect. If neither key works, investigate the vehicle-side systems such as the EIS/EZS, steering lock, power supply, and communication. If the key does turn but the cluster sits on a message instead of starting, that is a different fault: see COMPUTING DATA PLEASE WAIT.

Questions from Mercedes-Benz Owners

The dash lights and radio come on, but the key will not turn to start. What is wrong?

If the dash wakes up, the EIS (ignition switch module) has power, so the usual causes are the electronic steering lock not releasing, a key the EIS no longer accepts (dead key battery or failed transponder), or the EIS itself. Try your spare key first. A full system scan with a scanner such as the UCAN-II Pro narrows it down, because steering lock faults and key recognition faults are stored in the EIS and steering lock modules.

The metal key blade opens the doors, so why will the key not turn in the ignition?

The metal blade only works the mechanical door lock. The ignition needs the key’s electronic signal to be accepted by the EIS before it lets the key turn. Replace the key battery and try the spare key. If the spare works, the original key has failed. If neither key works, the fault is on the car side: the EIS or the steering lock.

My key stopped working after the car wash or after it got wet. Can it be saved?

Often, yes. Open the key, dry it completely, clean any green corrosion off the battery contacts and the circuit board with isopropyl alcohol, and fit a new battery. If the spare key still starts the car, the problem is only the wet key, and a new key can be ordered from the dealer with your VIN.

On my W204, W212 or Sprinter the key turns but the steering stays locked and nothing comes on. Is that the steering lock?

That is the classic sign of a failed ESL (electronic steering lock) on the W204, W207, W212 and W906 Sprinter. The fix is a new steering lock or an emulator, and either one has to be paired with the car’s EIS. If the EIS itself has failed, Module Medics clones EIS units so your existing keys keep working.

I replaced the engine, the ECU or the SAM and now the key will not turn or the car will not start. Why?

The engine ECU, the EIS and several other modules share immobilizer and vehicle data. A replacement or used module has to be coded and matched to your car before it will allow a start, which takes XENTRY at the dealer or a specialist. A used ECU from another car will not work until it is reset and adapted to your EIS.