The 1997 E420 has a DAS (drivers auth system). This system sends a signal to the key & must receive the correct info back from the key! EVERY key is numbered, and there is only 8 keys for the car! IF you lose a key & have to get a replacement key the dealer must know which key that is so that he can order the correct replacement. IF there is any question about this they should refer to SI MBNA 80/07 page 2. If they didn't specify any key they always will send key number 1.
YOU can check the key(s) number by removing the BATTERY cover

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