Mercedes EIS / EZS immobiliser key module


The Mercedes EIS (Electronic Ignition Switch) system is great in theory, the hardware being the switch is like a small socket in the dash to insert your key into – only, it is not a traditional looking key. It is more like a plastic lego-brick with what looks like an infra-red panel at the face of it which is inserted into the EIS socket on the dash. The key has a sequential code to ensure the vehicle is not stolen.
The vehicle will instantaneously carry out a sequence of test handshakes to ensure the correct key has been inserted. Each time the key is inserted, the EIS system will check the key and then ask the ESL (Electronic Steering Lock) system for verification that it accepts the correct key is being used. The ESL system will then look to see if the Engine ECU (Electronic Control Unit) agrees that this is the authorised key being used. – Then and only then, when all three control units agree it is the correct authorised key being used, will it allow the “Turn & Start” function to be activated.


The EIS module is factory fitted to almost all models and we’re getting units sent in right across the board from C-Class C220 Mercedes to ML’s and E350 Mercedes. You can expect the EIS module to be supported and installed on the following Chassis list:

W208 / W210 / W203 / W209 / W211 / W215 / W219 / W220 / W230


  • unable to turn the key;
  • unable to start the car;
  • can turn the key – but the car won’t start;
  • intermittent ignition key faults.

If the EIS unit becomes faulty, the “Turn & Start” function will be de-activated and the module will be locked for security purposes. This will stop the vehicle from starting. It is very unfortunate but also a very common problem as there are many reasons as to why these units are failing – most are due to human error. Some of the most common reasons are listed below:

  • Jump starting whilst the key is inserted;
  • Power surge;
  • Low battery voltage.

All of the above can cause memory loss to the EIS module and it cannot be repaired or re-flashed using the Mercedes STAR diagnostic system. At this point, you would have been advised to purchase a new unit which can work out extremely expensive when working out the total bill with programming and labour inclusive with VAT.

One of the main problems is the Motorola CPU chip which fails, even the newer batch of EIS modules have the same processor fitted so fitting a new module won’t solve the problem entirely, it’s how you look after the vehicle which plays a big part.



Here at O.G.S. Mechanics we can remanufacture your existing EIS Module unit and save you a lot of money. We thoroughly test & repair all EIS modules to ensure we locate the faults and rectify them completely. When the unit is returned to you, you will not be required to reprogram or re-calibrate the unit – simply plug & play.

After we have repaired your EIS module, you will be able to insert the key, turn the key and start the vehicle with no troubles at all. We do however recommend that you check the battery supply voltage and wiring to the EIS unit prior to installation to ensure there are no faults on the vehicle which could potentially cause failure of the replacement / repaired unit.