If we are talking about a 97 e420 then I would try another key. The security system DAS 2 uses a chip in the key energized by the ring around the ignition tumbler to do an electronic handshake. The batteries in the key and the remote control have no function in the DAS security function.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician