2004 CLK 320. I have two Smart Keys.
One key just stopped working - it won't turn in the ignition although it unlocks the car. I changed batteries with no improvement.
The other key still works fine, and will start the car even without the batteries.
Any help will be appreciated.

The key requires no batteries for starting the car, only for the remote unlocking functions, so this is normal.
For the one that won't turn in the ignition, key is either "bad" ie defective/broken, or is incorrect for the car. Both of these keys are original to the car? If you put the non-turning key in the ignition do you see a message in the info display in the cluster OR is there a small LED on the key that is lit when the key is in the ignition??
Gilly

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You didn't get any type of moisture into it did you? If so you might need to replace it. I did this back in July when my wife and I were at a 4TH July party and she got the key wet. I had a new one with in 5days. about $200.

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The key is original and worked fine up to today. There is no message in the cluster, nor a lit LED on the key when I put it in the ignition.
I guess the key just went bad. Good thing I have the spare.

I would get one soon, order an additional not a replacement the replacement wipes the other. MB screwed up and wiped my good one. Besides the new ones are better looking