Mercedes Benz owners may face a problem with Mercedes EIS / ESL Services preventing starting the vehicle. This is often caused by a damaged/defective key, a failing/failed Electronic Ignition Switch (EIS), or a bad Electronic Steering Lock (ESL). When a key is inserted into the EIS the immobilizer system will instantaneously carry out a check to see if it’s a correct “programmed” key that has been inserted. The EIS will verify with the ESL and Engine ECU to check that it is the correct key and if the units agree it will authorize the vehicle to be started.

First of all, if you have another key for that vehicle check if you experience the same symptoms with the spare key as it could have been caused by a defective key and not a failing Mercedes EIS / ESL module.
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Another option would be to get the EIS/EZS tested and either repaired or replaced with a unit from a donor car. A donor unit would need to be programmed to the vehicle so that the stored VIN is matched in all modules.
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We are able to offer testing for Mercedes EIS / ESL service as well as programming replacement donor units.