The ability to turn they key and start the car is disabled should the key sequence and EIS not match. Before the function to turn the car key and start the vehicle may be approved, three control systems (ESL, ECU, and EIS) should all confirm that the key is indeed the right one. Each Mercedes key is programmed with a numerical code that is specific to your car, and when you enter it into the starter, your vehicle performs a sequence of checks to ensure that it is the appropriate key. Do you own a Mercedes that has EIS or ESL? Both the Mercedes EIS (ESL Electronic Ignition Switch,) as well as the ESL systems are designed to keep your vehicle safe from theft.