Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
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Even though the lights come on when the door is opened you may still have a weak battery that can cause this here.If you can try to put a battery charger on the battery or a jump start then insert the key and see if it turns.If it does then the battery needs tested to see if it is faulty needing replaced.
If this does not work the most common cause is the key itself.When it fails to unlock the doors as well as turn in the ignition this is usually the key itself at fault as these are two separate signals from the key and both are not working generally pointing to the key at fault.
You would need to obtain a new key from a dealer (as this is the only option and it will come pre programmed ready for your vehicle) and they can order it in over night for you.You will need to show them vehicle registration and photo ID in order for them to order it in.
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