The key is the MOST likely problem!

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

You're asking for trouble by only owning one key!!! If the DAS portion of the key fails, you're stuck!!! The first step in diagnosing DAS is using a known good key. In my experience keys are at fault about half the time. This week alone we've had three '98 ML's with DAS problems. One faulty key, one bad AAM and one bad immobilizer. Order a new key before you do anything else. Even if it's not the problem you should have at least two anyway.