E320 Ignition key

I have a 1999 E320 wagon and one of the electronic keys has stopped working. It operates the remote locks just fine, but refuses to turn in the ignition
slot. I've put in brand new batteries, but that had no effect.
Any ideas before I resort to consulting with the dealer?
Thanks.
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More than likely, this is the original key and it is key #1. If yes, usually key #1 is used the most. Originally you got 2 keys, #1 and #2. Available keys are #3 thru #7. If you order a NEW key from the dealer, you will get the next one available: 3 or 4 or 5 etc. Once you go thru the 7 keys, you are dead for new keys. No more are available. You would then need to replace the EIS [electronic igition switch] and the ME [fuel injection] control unit and start over with 7 new keys.
The smart key has 3 tiers of codes. Each tier is rolling codes up to 200,000. Each time you start the car, the next code is used. The 1st tier is codes 1 to 200,000. The 2nd tier is codes 200,001 to 400,000. The 3rd tier is codes 400,001 to 600,000.
After 8 years, you may have used up the 200,000 rolling codes. The key is dead now for turning the ignition.
Go to your MB dealer and have them read the key number with the SDS of the key that is working. It may be #1 or #2. If it is #1 have the dealer order you a REPLACEMENT #2 key. NOT a NEW key. If it is #2, order a REPLACEMENT #1 key, NOT a NEW key.
I cannot stress strong enough about ordering a REPLACEMENT key for the bad one. In reality, there are 21 keys available for the smart key car. 3 for each key number. BUT you have to order them as a REPLACEMENT for an existing key. If you order a NEW key, you will use up the 5 available.
If you order a REPLACEMENT key for an existing good key, it will wipe out any lower codes that are remaining and render it useless forever.

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