Why doesn't the unlock light on my Mercedes key illuminate?

[OscarMarie]

It may be due to a failure in the key-vehicle sensing connection. Here are some suggestions when experiencing key-vehicle sensing failure: 1. Extract the mechanical key inside the smart key. Remove the keyless start knob cover and insert the mechanical key to start the vehicle. While the car can operate normally, the anti-theft alarm will keep sounding as the door security hasn't been deactivated. Visit an authorized dealership promptly to replace the battery or re-synchronize the smart key. 2. Check the built-in battery of the key fob - low battery power may require replacement. For emergency situations when the key battery is dead, press the key directly against the engine start button (maintaining about 1cm distance) while pressing the start button - this should allow the vehicle to start.